Some of the sketches are fairly straightforward -- they work like a seesaw, you might say. There's one rule, and they test and explore the rule until it leads to something surprising. Here's a sketch that works like a seesaw:
The rule that drives this sketch is that Steven Fry is, despite all appearances, Michael Jackson. If something seems off about that, well, it's just because Michael Jackson is a bit off -- Steven Fry's weirdness as a Michael Jackson only proves all the rumors about Michael Jackson true. He looks nothing like Michael Jackson because Michael Jackson has had all this surgery and turned himself into a white person, and if he's a poor dancer, well, it's only because he was always all flash and no substance anyway: just another spectacular showman. If he appears to lip-sync, that's because Michael Jackson lip-syncs.
The only real deviation from that rule comes at the end, when Hugh Laurie turns out to be lip-syncing as well. And this is only a typical comedic reversal, the cynical gesture that proves the original rule true; the logical culmination of Michael Jackson being a fraud is that everyone is a fraud, or at least everyone on television.
The basic SNL formula, of course, is that you take an incredibly specific rule or even sequence and then reiterate it to the point of absurdity, and then to the point of boredom.
The best sketches on the show usually change or mutate their rule several times. This one is a favorite:
In this sketch, the initial premise is seesaw-simple: the ignorant, conservative father is outraged that his son is learning about sex in school. The very polite, somewhat incredulous principle helps him make a fool of himself.
And yet it occurs to you that he seems to be making an unusually sweeping claim: not only that sex is a filthy thing to teach children, but that it is in fact a lie. The principle lets this pass once or twice but eventually presses him on this point. Which raises the question of how he's got a son in the first place -- a question he doesn't understand. This changes the entire sketch retroactively. The father presses on; he argues that free market principles should govern the school. Eventually, the principle asks what he means by this, what he wants done, and the premise of the joke changes a third time when the father asks for his son to be replaced with something of equal or greater value and the principle agrees.
Each change in the premise seems a natural mutation of what came before, and yet each change is so fundamental that it is also at least somewhat incompatible with what came before. So we predict the motion of the seesaw -- the evolution of that one rule -- and then there is the tremendous pleasure of discovering a new rule behind the old one, which transforms previous jokes and opens up new ones at what was previously the point of exhaustion.
Often in A Bit of Fry and Laurie the rules change abruptly at that point of exhaustion in order to avoid it. A gag threatens to grow tiresome, and they rupture the fourth wall, or otherwise disrupt themselves. A seesaw is a good toy. A better one can adapt to new modes of play.