His life would be like that of one of the Sims, except his speech actually has a script written by writers and not a random noise generator, and he has an actual scripted set of events that would always happen if you didn't intervene. Maybe to cover up how much this makes the player sound like a sadistic stalker, his life would always follow the path of saving his love, who will destroy the world.
Or you could think of it like taking the next step with Bioware's RPGs--instead of making decisions for a character that you have no direct control over, you are influencing events around this character instead. You're the tiny, invisible reaper that no one interacts with directly that somehow convinces Ashley to coat her womanparts with a neurotoxin that would be deadly to Shepard's unique physiology because he defiles God's plans. If the poison doesn't kill him (event is slightly randomized) it causes him brain damage, and hespends... therestofthegame. Talking. Like. Shatner!