I think we have to better define what we mean by improv in this context. Don't you think that many of your favorite actors would be able to attempt to portray any given role on the spot? Imagine if you were a director working with say Johnny Depp (I assume he doesn't have an improve background but fuck if I know just humor me) and you ask him to do a scene as an 18th century Louisianna landowner, don't you think he would make an attempt at portraying that role? Rather than saying no I don't have enough time to prepare I refuse. I mean, is their any famous, non-character, actor that doesn't have some sort of improv background? I hate to really pick on Kanye because it feels like picking on a little kid but have you seen his freestyle with Sway??? There is no freestyle, you could've done better than Kanye did, it was terrible. I don't hate him but I don't think it takes a "genius" to produce the music he makes, and that if you're going to play the part of a tough, ill-tempered rapper, then you best be able to back it up.