To the relative annoyance of movie-goers who like their pictures to hold up on the grounds of continuity, actors do - on occassion - like to opt out of appearing in sequels or follow-ups. It's rare, I know, but it happens. Something about trying to broaden their horizons and not taking an easy paycheck.
Pfft. That's to say, those vain, rich Hollywood types don't always sign on to reprise their roles for the sake of it, which is annoying if you're a movie producer with a potential sequel and you really, really want them to sign on and reprise their roles for the sake of it. Desperate times calls for desperate measures. Which basically translates as: "Okay, I'll replace you with somebody else," which is totally what happened in the 7 cases we've assembled for inclusion on this list (all of which actually paid off in unexpected ways). Not all of these actors were brought in as replacements because other actors refused to reprise their parts, though: sometimes death made things tricky, in the sense that the deceased are unable to "do acting" to an acceptable standard, apparently. In other cases, movie producers decided to change things up a bit without much explanation. Which is fine. But basically it meant that you sucked, re-cast Hollywood actor. Sorry about that.