Of the three things being discussed in this series, sequels are generally speaking the hardest to come to a vague defense of. And yet, they've been around for decades to say nothing of centuries. The biggest argument against them is that they seem to be nothing more than a money grab. But, truth to tell, one can say that of almost any commercial endeavor, especially in regards to movies. Movies are a business, after all, something most people forget.
Of course, I'm just speaking cinematically. You can take it back a lot farther than 1913. In fact, you can take the numbered sequel routine back to a guy whose name may or may not have actually been William Shakespeare. Seems he did a few Part 1 and Part 2 type plays back in the day. In fact, you can take it father back than that to the Greeks, with the Thebes plays by Sophocles and The Illiad and The Odyssey. "Yes, but those are classics" you may be saying. They're classics now, but they were pop entertainment--in particular Shakespeare's work--in their day.
Sequels proliferated in novel format once the printing press was invented. Accordingly, a number of said sequels were essentially repeats of the novel that preceded them. Sound familiar? This was done mostly to assert the author's ownership over the properties.
The point is, this sort of thing has literally gone on for centuries and isn't likely to change any time soon.
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Up until about the 1970s, however, sequels--or at least movie sequels--tried to tell somewhat different stories and have completely different titles. The sequel to the 1931 Frankenstein wasn't called Frankenstein 2, it was called The Bride of Frankenstein. While some of the nuts and bolts of the story were similar, it also wasn't a direct repeat of the first movie, either. Not even serials told the same exact story or numbered their sequels.
There were a couple of examples of sequels being numbered prior to the 1970s--Quatermass 2 from 1957 being possibly the earliest example--but starting in that decade, the floodgates started to open. While both The Godfather Part II and The French Connection II were both considered better than their respective first movies, they also seemed to help get the trend of numbering started. The Exorcist II, Rocky II and Jaws 2 followed by the end of the decade. Then came the 1980s.
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The trend was so bad by 1983 that Siskel and Ebert's "Worst of the Year" show for that year was dedicated to nothing but sequels. I have to admit, there were some pretty wretched sequels unleashed on the public that year. They highlighted such rotten sequels as The Sting II, Staying Alive--the sequel to Saturday Night Fever and the only film on the list of sequels to not have a number--, Smokey and the Bandit 3, Jaws 3-D (which I kinda like even though it is a bad movie), and Amityville 3-D. At the end of the show, they implored audiences to not watch any sequels unless they said so.
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Part of the problem, which again goes back to the 70s, is that following the law of diminishing returns, sequels got lower and lower budgets as the series went on. The original Planet of the Apes series is a prime example of this. The excellent make up jobs on all of the apes in the first film gave way to bad Halloween masks in later entries. The diminishing budgets may not be as evident in comedies like the Revenge of the Nerds series, but it comes up front and center in an effects heavy series like Superman. The budget and effects for the notorious Supergirl and Superman IV were so bad that they killed the series until the 2000s.
Lately, however, that doesn't necessarily follow. Movies are trying to put more money into the sequel, to make it bigger and more spectacular than the film that came before. Sometimes that pays off. A fairly surprising source of good sequels nowadays seems to be in the comic book genre. With the exceptions of a few notable missteps like X-Men III--a movie I truly regret seeing--the comic book genre has really stepped up. The latest Thor and Iron Man movies were, if not better than their predecessors, at least as good as. Spider-Man 2 and X2 were also better sequels. Christopher Nolan's Batman trilogy got better as it went along. In the coming months we'll have sequels to both Captain America and X-Men that both look like they should be good. Again, like Sci-Fi movies, the thing that these newer comic book movies have going for them is their ability to tell different stories in each film. That helps make the movies at least seem fresher, or at the very least, not make us think "didn't we see this story already?".
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So, at the end of the day, should you listen to Siskel & Ebert's advice from 30 years ago? No sequels unless directly told otherwise. Not necessarily. I still watch sequels and in a few weeks will be happily dropping money to see the second Captain America. I'm fairly sure it will make a mint, too. So long as sequels make money, sequels get made no matter how good, bad, or indifferent. That's the way it's always been, that's the way it is, and that's the way it's always going to be.