Case in point: The Wrath of Khan. You don't actually need to have seen "Space Seed" to enjoy that film--and I suspect that most moviegoers hadn't. (Or, at best, had maybe dim memories of Ricardo Montalban guest-starring in old Trek episode.) As for Casablanca, I've always thought that the weakest (and most unnecessary) part of the movie is the flashback to Rick and Ilsa's romance in Paris. We didn't really need to see that acted out; the look on Rick's face when Ilsa walks into his bar years later pretty much tells us everything we need to know . . . Let's just be thankful that Raiders of the Lost Ark didn't feel obliged to explain exactly how Indy and Marion broke up years ago.