As part of modern video game projects, game designers do all of the following:
Plan game systems around intended gameplay style: As an example, how does the player-character health model work? Does damage simply subtract points of health (and does healing simply add back points of health), or does health regenerate? Does the player-character’s health never drop below 1 point on any single attack (as in some games that try to facilitate dramatic moments, like City of Heroes, Call of Duty or Mercenaries)? Game designers plan the player’s movement package, the advancement of his powers, his weapons and tool set, etc.
Develop fiction: As an example, if we’re working on a caveman game, members of the design team (with fictional talent) might work out a plot (or work out a plot with a contract writer) that puts the player in the role of a young caveman, trying to move his tribe to a new hunting ground. The strike team responsible for fiction will work out a series of missions or objectives that support this plot.
Tune gameplay ecology entities: As an example, a designer might have to enter physics values for a set of vehicles, making a truck drive and turn slowly, while a sports car accellerates and corners sharply. The truck might sustain more damage before bursting into flames. Similarly, a game designer might take the AI system (written by a senior engineer) and set up interesting behaviors by tweaking the AI parameters of each enemy type.
Build crude gameplay environments: As an example, game designers will rough out a space, like a junkyard, planning the player’s flow through the map/mission.
Script gameplay: As an example, if junkyard dogs are supposed to rush out at a given moment, some (technical) game designer sets up some trigger logic to initiate this event.
Convey cohesive vision: As an example, after analyzing tons of competition, a game designer might look at the game holistically and try to articulate the overall tone and nature of the desired experience, then communicating this to team mates.