How to let systems create emergent play

1 The bottom line

Build your world from a few consistent rules that every object, character and AI shares, and let them collide. Players then invent solutions and stories you never wrote, and the game stays fresh long after the scripted content is spent.

2 Key takeaways

• Write rules that apply everywhere, not one-off scripts. Players can only experiment with rules they can learn and trust.
  • Mark Brown, Game Maker's Toolkit4:08: in Thief: The Dark Project, rope arrows attach to any wooden surface, while Thief (2014) only lets you grapple beams marked with white rope.
  • Mark Brown, Game Maker's Toolkit0:35: in Breath of the Wild, rain touches climbing, stealth, combat, fire, wildlife and NPCs.
• Let every entity listen for inputs and broadcast outputs. Encounters then arise when entities meet, with no author in the loop.
  • Mark Brown, Game Maker's Toolkit0:45: in Far Cry, a tiger that meets a guard mauls him without any script.
  • Mark Brown, Game Maker's Toolkit2:37: in The Sims, objects advertise what they offer, so designers can add hundreds without touching the AI code.
• Give players a goal and tools, not a method. Solutions the player invents feel earned, and they differ from run to run.
  • Mark Brown, Game Maker's Toolkit2:41: Thief says to steal the sceptre from Lord Bafford's Manor but never says how.
  • Mark Brown, Game Maker's Toolkit5:31: in Hitman 3's Berlin, any five of eleven targets finish the mission, giving over 400 combinations.
• Let characters use the same systems and keep surviving, so stories form. A story needs a partner who is still there next time.
  • Mark Brown, Game Maker's Toolkit11:41: Shadow of Mordor builds its plot encounter by encounter from surviving enemies, a changing hierarchy and linked orcs.
• Plan for the cost of freedom. Unforeseen interactions and exploits arrive whether you planned for them or not.
  • Mark Brown, Game Maker's Toolkit7:03: when a one-way platform met a held magnet, he had to decide the outcome himself.
  • Mark Brown, Game Maker's Toolkit8:25: Tears of the Kingdom testers built perpetual motion machines that had to be nerfed.

It's constructed at runtime, encounter by encounter, using your interactions as narrative brickwork.

Mark Brown, Game Maker's Toolkit

2.1 Decide how much you hand over

Choose where your game sits between hand-made content and a sandbox, then commit to giving players goals and toys.

• Plot your game on a passive to active line. See Mark Brown, Game Maker's Toolkit7:51.
  • In a passive game, the designer hand-crafts content and the player follows.
  • In an active game, the designer builds a sandbox, hands over toys, sets a goal and lets the player get on with it.
  • Call of Duty and The Last of Us sit at the passive end. Minecraft and Factorio sit at the active end. Skyward Sword is passive and Breath of the Wild is active. Neither end is inherently better.
• Give open-ended problems instead of one intended answer. See Mark Brown, Game Maker's Toolkit7:47.
  • Classic Zelda puzzles have a single solution. Tears of the Kingdom gives tools and lets players find their own.
  • Where Breath of the Wild let you climb a mountain, Ultrahand lets you fly to the summit in seconds with a goblin glider.
  • Dohta says the focus is on letting players do what they think they can do, not how the designers want them to play.
  • Mark Brown, Game Maker's Toolkit11:54: Nintendo added more radical toys after watching how players used the Breath of the Wild sandbox. Ascend began as a debug tool for skipping content.
• Tell players the goal, not the method. See Mark Brown, Game Maker's Toolkit2:41.
  • Give a clear objective, a large open space, a few routes for different playstyles, and some hints.
  • Mark Brown, Game Maker's Toolkit22:37: Deus Ex can be played as a shooter, a stealth game, an adventure game or an RPG. Warren Spector wanted the game to reflect the player's creativity, not the developers'.
• Never promise freedom and then remove a play style. See Mark Brown, Game Maker's Toolkit8:39.
  • Give each style a valid, enjoyable route through every level, and test the game in every style to find brick walls.
  • Deus Ex: Human Revolution forced boss fights even on non-lethal builds. The Director's Cut redesigned the bosses.

2.2 Write rules that hold everywhere

Define traits globally so players can learn a rule once and apply it anywhere.

• Build from global systems instead of one-off scripts. See Mark Brown, Game Maker's Toolkit3:36.
  • Every alarm post works the same, every torch can be extinguished, and every door comes from one generic mould.
  • Players learn the rules and trust them instead of memorising special cases.
  • Mark Brown, Game Maker's Toolkit4:08: the systemic version lets players experiment, while the scripted version tells them where to act.
• Build the world from interacting properties. See Mark Brown, Game Maker's Toolkit5:26.
  • Breath of the Wild uses wind, fire, electricity and gravity, and players exploit their relationships to solve puzzles, beat enemies and bypass obstacles.
  • Fujibayashi wants players to relish moments where they think they are a genius.
• Make one system reach into many others. See Mark Brown, Game Maker's Toolkit0:35.
  • Breath of the Wild's rain makes rocks slippery, hushes footsteps, boosts electrical damage, turns fire arrows into normal arrows and puts out campfires.
  • Rain also brings out certain creatures and sends people running for cover.
  • Mark Brown, Game Maker's Toolkit5:43: volcanic zones burn wooden weapons and lakes are ice-cold, so the environment becomes a threat to plan around.
• Get big effects from simple local rules. See Mark Brown, Game Maker's Toolkit7:07.
  • For fire, lay an invisible grid where each cell has hit points. A flammable cell that reaches zero ignites and damages its neighbours.
  • Give cells a lifetime, vary hit points by environment, and weight damage toward the wind.
  • Mark Brown, Game Maker's Toolkit12:52: Red Faction: Guerilla gives bricks a weight and a strength and asks whether each layer can hold the layers above it. Teardown and Bad Company 2 use rules that feel arbitrary by comparison.
  • Mark Brown, Game Maker's Toolkit13:42: falling debris can crush other buildings, though the stress calculation makes collapses slow.

2.3 Let characters and AI use the rules too

When enemies and objects take part in the same systems, players can plan around them, and the world acts without them.

• Give every entity inputs to listen for and outputs to broadcast. See Mark Brown, Game Maker's Toolkit0:45.
  • When an input and an output match and the entities can see or touch, a rule fires.
  • Far Cry's tiger listens for the player, bait, fire and enemies, so meeting a guard triggers a mauling with no script.
  • Mark Brown, Game Maker's Toolkit2:37: in The Sims, a bed says it gives +10 energy and a toilet offers +20 bladder. The Sim ranks every advertised option by its current needs.
• Let enemies be aware of more than the player. See Mark Brown, Game Maker's Toolkit3:19.
  • When entities know about each other, players can plan indirectly instead of just shooting.
  • Far Cry lets you release a caged animal onto nearby guards.
  • Mark Brown, Game Maker's Toolkit2:43: in Watch Dogs 2, he led the cops into gang territory, hid on a rooftop while the factions fought, then slipped away and lost his wanted level.
• Let AI use the game's systems so it looks smart and can be exploited. See Mark Brown, Game Maker's Toolkit7:47.
  • Breath of the Wild enemies pick up dropped weapons, light their clubs, kick away bombs and throw other monsters.
  • A Bioshock enemy runs to a health dispenser, and the player can trap that dispenser. Players can also make an enemy fight for them in Prey, or trick one into attacking a Cucco in Zelda.
• Give AI goals beyond attacking the player. See Mark Brown, Game Maker's Toolkit10:33.
  • In Rain World, animals hunt for food and fight rivals over territory.
  • In STALKER: Shadow of Chernobyl, you can stumble on a battle that arose from two factions bumping into each other.
  • In Waking Mars, AI critters live and breed in ecosystems you build, even while you are elsewhere.
• Score options by current need, then choose loosely. See Mark Brown, Game Maker's Toolkit3:18.
  • Multiply an object's promised score by a factor based on the motive. A rested Sim damps the bed's score and an exhausted Sim boosts it sharply.
  • Give each need its own curve so urgent needs win, as in Mark Brown, Game Maker's Toolkit4:35.
  • Pick at random among the top scorers so the Sim stays imperfect, as in Mark Brown, Game Maker's Toolkit6:34.
  • That imperfection means Sims cannot meet every need perfectly, which gives the player something to do.

2.4 Hand players tools that combine and reshape

Interlocking tools invite experiments, especially tools that change the space or the AI's behavior.

• Let abilities combine so players invent uses you did not script. See Mark Brown, Game Maker's Toolkit3:34.
  • In Prey, the Lift Neuromod sends heavy objects into the air, and Gloo pins them to walls as floating platforms.
  • Dropping them on enemies works in theory, but he spent about 30 minutes and managed only a rough result. Not every experiment needs to pay off.
  • Mark Brown, Game Maker's Toolkit5:42: tools that add objects need firm limits. Minecraft uses a voxel grid, Dreams has a creation budget, and Shadow Complex's foam gun works in two dimensions.
• Give tools that reshape the space to suit the player's plan. See Mark Brown, Game Maker's Toolkit6:38.
  • Thief's water arrows put out candles to make darkness, and moss arrows make silent tiles.
  • Splinter Cell offers a rarer version by letting you shoot out the lights.
  • Mark Brown, Game Maker's Toolkit1:26: in Space Invaders, clearing the outer aliens means the formation travels further before it drops, so removing threats manipulates the play space.
• Use distractions and gadgets to pull the AI where you want it. See Mark Brown, Game Maker's Toolkit2:42.
  • Sight lures include a Playboy magazine, a decoy and a dropped weapon. Sound lures include the sonic Batarang, the sticky camera speaker and rocks.
  • Hitman even lets the world do it, such as an overflowing sink.
  • Mark Brown, Game Maker's Toolkit3:05: Mark of the Ninja's Terror Dart makes an enemy panicked and paranoid, so they may kill their own teammates.
• Let optional tools make challenges easier. See Mark Brown, Game Maker's Toolkit5:23.
  • In A Link Between Worlds, some enemies and puzzles are easier if you have rented other items.
  • It is not Breath of the Wild levels of improvisation, but it gives players room to experiment.

2.5 Let stories grow from the interactions

Give characters memory, rank and relationships, and the story assembles itself from what the player does.

• Build the story at runtime from encounters, not a script. See Mark Brown, Game Maker's Toolkit11:41.
  • Most games, even with branches and endings, are plotted in advance like a choose-your-own-adventure book.
  • Give every fight an ending other than the enemy's death. In Shadow of Mordor, the player can flee, orcs can retreat, and a downed orc may escape, as in Mark Brown, Game Maker's Toolkit4:36.
  • Maku the archer keeps downing Mark with poisonous arrows but stops short of killing him, taunting him to show some pride of Gondor.
  • Mark's run produced a story nobody wrote: sword-breaker, Stitch, ally, bodyguard, then new Overlord.
• Use a hierarchy and a web of relationships as the frame. See Mark Brown, Game Maker's Toolkit8:37.
  • Shadow of War uses overlord, warchiefs, captains and soldiers. Killing Talion promotes an orc and shaming a warchief demotes them.
  • Orc missions resolve on their own, so the world keeps moving without the player.
  • Mark Brown, Game Maker's Toolkit11:13: orcs can be bodyguards or blood brothers, and a recruited Ar-Karo later saves Mark from death. Recruits do leave the map, so that bond is thinner than the rival one.
  • Mark Brown, Game Maker's Toolkit11:21: a relationship matrix would let kindness to someone's best friend raise your odds with them. Griftlands has a bit of this, where allies you paid off buff your stats in negotiations.
• Show a hidden stress system as a number, but keep its consequences unpredictable. See Mark Brown, Game Maker's Toolkit3:15.
  • In Darkest Dungeon, damage, darkness, a dying teammate and monsters all raise stress. At 100, the hero gets an affliction such as stealing treasure or skipping turns.
  • Let breakdowns sometimes become virtues. Pressure can occasionally make a hero overpowered, as in Mark Brown, Game Maker's Toolkit4:02.
  • Because the team acts like humans rather than chess pawns, the game produces emergent stories without scripting them.
• Let earlier characters and invaders change your world. See Mark Brown, Game Maker's Toolkit17:20.
  • In Prey: Mooncrash, you repair the portal as the engineer and hack it as the custodian, then escape as the volunteer.
  • It also works against you. A used escape pod is gone, and items scavenged by one hero are gone for the next.
  • Mark Brown, Game Maker's Toolkit6:26: Deathloop's Juliana invades and places mines that were not there in the last five playthroughs.

2.6 Decide, cap and keep the machine running

Systems collide in ways you did not plan, so choose outcomes, keep the good accidents and stop the ones that break the game.

• Decide the outcome of every unforeseen interaction. See Mark Brown, Game Maker's Toolkit7:03.
  • Combining mechanics creates cases nobody planned. Either the design resolves them or you get a glitch.
  • Mind Over Magnet had a one-way platform that blocks the magnet but not the player. He chose to rip the magnet from the player's hands.
• Keep accidents that add depth, and fix ones that only exploit glitches. See Mark Brown, Game Maker's Toolkit4:07.
  • Psyonix added a speed boost by applying a force to the back of the car. Testers used it in mid-air, and it became Rocket League's aerial movement.
  • Keep unintended solutions only if they are clever, skilled and obviously off-script, as in Mark Brown, Game Maker's Toolkit12:17.
  • Players threw the magnet to block a laser and slipped past, which he keeps. Fudging into a collision edge to grab a magnet has to go.
  • Mark Brown, Game Maker's Toolkit18:28: Team Cherry designed in sequence breaks such as nail bounces replacing the Monarch Wings. A stuck spot in Kingdom's Edge still had to be patched.
• Cap the exploits that break the game and design for recovery. See Mark Brown, Game Maker's Toolkit8:25.
  • Fujibayashi's team spent a long time finding limits that allow real creativity while keeping the game working as a game.
  • Build levels that find their way back to a working state, as in Bjarne Kristiansen and Håkan Almer, IO Interactive7:26.
  • Berlin's targets do not call for backup or go into lockdown, so one alert cannot collapse the level.
• Let failures be funny. See Mark Brown, Game Maker's Toolkit6:21.
  • Ultrahand machines can run away and wooden tanks can burn. Nintendo accepts this as long as players find the failures funny.

2.7 Where designers disagree

Freedom has costs, and the right amount of predictability, scripting and polish depends on the game.

• Predictable worlds versus surprising ones.
  • Mark Brown, Game Maker's Toolkit8:58: Hitman 2 sets targets on fixed schedules, so players who know what will happen and when can build a plan and trust it.
  • Mark Brown, Game Maker's Toolkit6:26: Deathloop adds an invader because fixed levels become perfectly predictable.
• Welcome every cheat, or only the clever ones.
  • Mark Brown, Game Maker's Toolkit7:56: a clumsy cheat is still fun, because the player feels like the only one who found it.
  • Mark Brown, Game Maker's Toolkit12:17: an off-script solution should take skill and be obviously a clever way of beating the designer.
  • Mark Brown, Game Maker's Toolkit15:42: player-driven solutions can also lead to repetitive answers, cheesy exploits and outright cheats.
• A clever system is not enough.
  • Mark Brown, Game Maker's Toolkit10:49: Watch Dogs: Legion's expensive recruit system is largely meaningless because missions and story rarely need it.
  • Mark Brown, Game Maker's Toolkit7:07: Warren Spector would rather make something an inch wide and a mile deep. Mankind Divided falls short because time stands still between missions.
• When to script instead.
  • Mark Brown, Game Maker's Toolkit10:00: L.A. Noire sensibly tells you where to go, while other detective games build dead ends with procedural generation or fake databases.
  • Mark Brown, Game Maker's Toolkit3:30: Berlin keeps a few scripted kills, but most targets die to guns, poisons, distractions and traps from the systemic sandbox.
  • Mark Brown, Game Maker's Toolkit9:24: immersive sim works better as an ethos than as a checklist of features.