For the most part, the actors who are appearing just sign autographs. However, once each weekend, ONE of the actors appearing that weekend will usually do a moderated Q&A session in one of the studio theatres. Last year when we were there, the one who did the Q&A was Silas Carson; it was rather interesting to hear the discussion of how he worked out the Viceroy's strange accent. DS doesn't do autographs, but he wanted to hear that Q&A session; he is fascinated by the process they go through to make the films.
The costumed characters who appear are pretty much the same crew the whole time. There may be smaller or larger groups of Stormtroopers about, though; many of them are members of a local costume group (not WDW cm's) and there tend to be more of them around early in the morning; I don't think they get paid to be there, and that armor is HOT. Try to be at the studios at opening one of your days; the troopers stand on top of the gate and make a lot of cracks about people waiting to get in; it's pretty funny.