6. Your Cast Will Age Contrary To Your Film
Time indeed marches on. And for sequels, this only requires your cast to show age in regards to fictional time past. Most films in this regard choose to only advance time relative to actual years past or make sure to film as consecutively as possible. (Harry Potter Films are a great recent example - They really only could slow down after their actors' puberty, and even then it was pretty quick film scheduling.) Really, as long as your cast is still able to pull off their role, you can roll out sequels indefinitely. (In theory, anyway...) Prequels immediately create a problem as they require your cast to be younger. Sometimes DECADES younger. Immediately, you have the task of casting someone who'll be believable in an iconic role. Possibly replacing an actor or actress that is pitch perfect or regarded as such. "The Hobbit" escapes this to a degree with Christopher Lee, Ian McKellan, and Hugo Weaving as their characters were easy to mask under makeup, robes, and (lighter shaded) beards. And it seems rather good, Martin Freeman's casting? No one seems to mind him being a young Ian Holm. However, it seems that Elijah Wood (again, surplus to the film, no less) is less well regarded in his appearance - is it really his fault that almost 10 years have passed? These issues make it almost impossible for some to overlook and immerse themselves in a film. I watched "X-men: First Class" accepting Michael Fassbender not only as a stong portrayal of Magneto - but also because I was able to imagine him growing older, and into Sir Ian McKellan. Fair? Not really. Unavoidable? Absolutely.
Guilty Parties: Oh, just about any Prequel has this issue too. I mentioned "X-Men: 1st Class" and "The Hobbit" ...and I was HAPPY with those in regards to their castings. It's not always done so well; Gaspard Ulliel is fighting a losing battle in "Hannibal Rising" as the titular character; You just can't forget Anthony Hopkins. This is a particular issue that made me rule out low budget/direct to video releases; I'd've been listing films all day. But effectively, special notice has to be given to "Twin Peaks: Fire Walk with Me". Released following the show's run, this prequel film largely allows its cast to return....and defiantly challenges you to believe that half of them are supposed to be in High School.
Exception: Come on down, Star Wars! In addition to largely just using voice talent for earlier roles (Excepting Obi-Wan Kenobi, who Ewan MacGregor did play respectably) Star Wars' prequel success (Just about their only one in this article!) is the ability to reuse Ian McDiarmid successfully as a younger version of the Emperor from "Return of the Jedi". In fact, I've seen it reasonably argued that he comes out of the prequels as the one character who actually gets properly fleshed out. Which is great....except the character who was supposed to get that treatment was Darth Vader. Whoops.