@Edo Levine addressed that issue in his RPS interview:
"Our action sequences are much more custom, much bigger. We have this opportunity to create this great level of consistency, like the scene in the doorway where she puts your hand around her neck, that’s so specifically animated. But then when she is in the world her actions have to be roughly on a par with that, consistent with that, even though we have no control over what the player is doing, or where he is going to be. It’s really complicated and really challenging, but we thought “it’s time to make it happen."
[...]
"Okay, so, because we have these very specific moments – we know you’re going to go through that door and Elizabeth and that’s a scripted moment – then when you’re just walking around the city, we don’t know what the player is going to be doing. So the rest of the stuff in the store’s demo is not intended to be scripted in the same way – like the part where she puts on a Lincoln mask. She’s just goofing around. She’s using her own systems to do that. We build in these little moments, like the Lincoln head moment, and we build them where they can happen in a variety of play-throughs, and we have to have some redundancy, and we didn’t want them to repeat, or to stack up. So we had to build this system where the game is watching: Has Elizabeth done anything interesting for a while?Is there anything interesting that she could do here? Ah, there’s this Lincoln head, let’s do this. It’s walking an interesting line between scripting and emergence. Because you can’t just have pure emergence, you have a semi-emergent system where things like the Lincoln head can happen, if the situation is right."
More here: http://www.rockpapershotgun.com/2011/07/07/bioshock-infinite-gillen-vs-levine/