As any of a vast number of Hollywood films prove it's possible to get great actors for sometimes the most fleeting roles. It's all about paying them and the billing. Sometimes like you said they will do it just to be a part of a successful series like Batman. Or because they want to do something different. I've heard of lots of stars doing kinda minor roles in kiddie type movies (and Star Trek is kind of kiddie stuff) so their kids could see them in a movie. I'm sure if Bardem has munchkins around he's not letting them watch daddy in No Country for Old Men. And there is the epic war movie deal. Just look at The Longest Day and A Bridge Too Far. Dirk Bogarde, James Caan, Michael Caine, Sean Connery, Edward Fox, Denholm Elliot, Elliot Gould, Gene Hackman, Anthony Hopkins, Hardy Kruger, Laurence Olivier, Ryan O'Neal, Richard Burton, Robert Mitchum, Sean Connery, Henry Fonda, Red Buttons, , Peter Lawford, Gert Frobe, John Wayne, Irina Demick, Curt Jurgens and Robert Wagner, Robert Redford, Maximillian Schell, Liv Ullman, (Sean Connery is on there twice because he is the only person to appear in both). They got all those people to be in just two movies. A lot depends on the project, and there has never been a 'minor' StarWarsTrek project.