Drop-In Co-op Campaign

Story content has to hold more than one player. Checkpoints must handle both being alive, encounters scale in numbers or health, cutscenes need to account for a second character, and a joining player has to be inserted without restarting anything. Simplest implementations put both players on one screen and one save file; later ones connect over the network and let a guest keep progress earned in somebody else's game.

Bungie shipped Halo: Combat Evolved in 2001 with its full campaign playable in split-screen cooperation, treating that as a default rather than a bonus mode. Console hardware sitting in a living room made the assumption reasonable, and the game's encounter design, built on open arenas and vehicles, tolerated a second unpredictable participant far better than a corridor shooter full of scripted sequences would have.

Playing a story with a friend turns out to be among the most requested features in the medium and among the most expensive to provide: every system is tested twice, difficulty tuning becomes ambiguous, and narrative games struggle to explain the second character's existence. Studios have answered with scaling, with fiction that assumes a partner, and increasingly by designing the campaign for cooperation from the beginning.