BioShock, and it's shaping the way he directs the voice actors -- in person for the first time -- in BioShock: Infinite, Levine told Gamasutra.
Writing Elizabeth and Booker, Infinite's main characters, was a completely different process than writing BioShock's antagonist, Andrew Ryan, Levine said. "I always had Ayn Rand in my ear while I was writing him, and she is quite articulate in her viewpoints. So he was a pretty easy character to write, for me," Levine said.
"Booker and Elizabeth, because there's a very different constraint set, because I haven't done this kind of writing for a game before, where you sort of have all this dynamism with a character you're walking around the world with, that you're speaking to, as Booker... just the mechanics of it!"
Levine said he was inspired by the easy banter Naughty Dog placed in Uncharted, and he saw how it could transfer to a period piece. As he describes them, Levine's characters are unique to their time period and his own imagination: "Elizabeth is a person who sees nothing and wants to see everything, and Booker is somebody who's seen everything and wants to see nothing. They're at opposite ends of the spectrum."
The full (and long) interview is here, if you're interested in details about how Levine makes women weep.