Not a Hobbit
Explorer
I've done something like this, in the past. What I did was to just deal with each player, as they came in. So, just describe the room (box text). When the first person comes into the room, tell them that an old lady comes out from behind a bookshelf and greets them (or whatever). As long as they're the only one who engages, let it go. As soon as another player interacts with the scene, in some way (say, picks up a book), narrate the entity's reaction with a brief description ("The scholarly young man turns his attention from Bob to you and says, 'Please be careful, those are old and fragile.'"). You can do the same sort of thing, even if only one PC is interacting with it, too.
The first time or two, the players will probably assume they missed the description of someone else being in the room, or you made a mistake. Maybe one of them will even say, "I ask the little girl XXX," and you can say, "There's no little girl, just a talking cat." It'll be a bit confusing, which is the point, but they'll eventually either catch on or ask direct questions of you (as GM), at which point you can be explicit about what each of them sees.
Thanks everybody for some great suggestions. I think this is the way I would love for it to happen, but I'm afraid I'll confuse myself even more than my players

When the movie comes out, though, this scene is gonna be frickin' awesome!
Thanks again. I've got a couple of weeks to figure out what to do. I'll let you all know (if anyone cares).
--Scott