[NaS]

First off, actors (and all other ped-like entities) don't change their collision model when performing an animation.
That means that the actor basically still stands (collision-wise).

Second, the chair might have an imperfect collision model, if it's not detailed enough there is no chance to place an actor on it. Well, it's maybe possible in some cases but it might look different for every player.

Find a chair without collision, that will work. If it looks ugly, you can retexture it.