I was gonna say, I associate him pretty strongly with Professor X! But then I haven't watched Star Trek either, so that might help. It's a little like David Boreanaz though - he got two iconic roles and they're the only ones I can ever imagine him in. At least one of the roles spanned two different shows. :P

There definitely seems to be a point of no return with these things... Although in very rare occasions I believe some actors have broken out and started getting other roles and changed the view of them. Must be hard though, and rely a lot on luck as well.