Make your characters likeable.
First and foremost. Your show can be the silliest, dumbest thing on TV, but as long as people enjoy watching the characters, they’ll probably still stay tuned. The best example of this right now is indeed Sleepy Hollow, as Tom Mison’s delightfully bitchy Ichabod Crane is just wonderful to watch, no matter how absurd everything else is. Agents of SHIELD, plot-wise, has been pretty ho-hum, but Clark Gregg is carrying most of the show‘s weight on his shoulders through his charm as Agent Coulson. Silas Weir Mitchell’s Munroe as definitely been the jewel of Grimm of both seasons, while star David Giuntoli has slowly been coming into his own personality, and he’s definitely the best part of the series. And a major factor in making your characters likable is…
Have the characters take the ridiculousness seriously and realistically.
You can get away with pretty much any plot as long as the viewers feel like the characters are responding to it like real people actually would. For many shows, that means a profound bafflement, like Anthony Edwards seeing the frozen corpse of a Nazi officer who looked exactly like him in Zero Hour, or Ichabod Crane waking up in 2013 in Sleepy Hollow. Real people would be boggled by these things, and to immediately brush them off is simply dumb. The flipside to this is that at a certain point, characters do need to accept the ridiculous of their situation, or they actually become stupid by ignoring their reality. Seeing is believing, and most humans, when faced with enough facts, will accept almost anything. Remember how annoying it was in The X-Files when Scully would never, ever believe a supernatural cause for their cases, even though she would have arrested Bigfoot last week? Realistic characters will understand that their situation is absurd, but they'll also admit it to themselves and then cope with it. This also means…
Don’t make your characters stupid.
The show may be ridiculous, but that doesn’t mean your characters have to be. Certainly, if you have a show like Zero Hour which is about a secret society designed to steal the cross Jesus was crucified on in order to clone Jesus to bring about Armageddon, then you’re going to have some explaining to do on the way. But in a show like Grimm — well, in Grimm, Nick has been fighting fairy tale monsters called Wesen for two full seasons, and yet for some reason 1) he never, ever expects one of the countless weird crimes he encounters to have been committed by a Wesen until something else weird happens, and 2) he’s apparently never, ever heard a single fairy tale before, meaning every single new Wesen needs to be explained to him in detail, even when it’s something pretty well known, like Rumplestiltskin. I mean, even if he somehow grew up without ever once hearing about Rumplestiltskin — which seems unlikely — shouldn’t Nick at some point sit down and read a copy of Grimm’s Fairy Tales in order to be a better Grimm? Note: Sleepy Hollow managed to avoid this issue, because in the show Ichabod Crane is real, but Washington Irving’s story is not. This has avoided countless problems.
Play by the rules you’ve set.
Your show can be as completely insane as you want it to be. You want to take the legend of Sleepy Hollow and have the Headless Horseman be one of the Four Horsemen of the Apocalypse and also there’s a demon named Moloch in charge who’s decided a small town in New York is ground zero for Armageddon? That’s completely fine. But once you establish what you’re working with, you’ve got to play by the rules you set, because other you break the covenant you have with the viewer. They’ve agreed to suspend their disbelief for your premise — there’s a little bit of wiggle room there — but something that directly contradicts what’s come before is a dealbreaker. For instance, as insane as Sleepy Hollow is, at no point are aliens going to be able to show up. A good example of a show that fucked this up is Alias; it was a very ridiculous — and fun — spy show that required a fair amount of disbelief, with ridiculous double-crosses, insane plans, and assassins modified to look like other people. But then Alias somehow became about zombies, and it was dumb. Not that there can’t be shows about zombies, obviously, but you can’t throw them in season 4 of a non-supernatural show and expect people to keep watching.
Perhaps even more important that having likable characters is pacing. Lethargy is death to the ridiculous show, because any lulls give the viewers time to realize what they’re watching is kind of dumb. Sleepy Hollow avoids introspection of its completely ludicrous plot by keeping the mysteries and action coming continually. Grimm suffers because it takes so long for the Grimm team to figure out what’s going on, and then how to fix it, and then executing their plan. Zero Hour was pretty much as ludicrous as Sleepy Hollow and didn’t have likable or smart characters, and got away with it purely because the show moved at a lightning fast pace, as Anthony Edwards and crew kept jetting around the world, solving old silly mysteries while always introducing more. And a huge part of this is…
Don’t focus on unnecessary shit.
I know you have a large cast and you feel compelled for them all to have something to do every episode. I know that logistically, your star can’t appear in every scene of every show without committing suicide from overwork. But if you can’t tie a subplot back to the main story — or at least use it to explore the setting of the show — you need to cut it. Not to pick on Grimm again, but half of Juliette’s scenes last season could have been cut without losing anything. Not that it’s a scifi show, but Dexter was horrible at this — think of all the time spent with LaGuerta’s troubles, or Quinn, or, most useless of all, Masuka and his daughter. None of those scenes mattered at all to Dexter, the character or the show. They went nowhere, and never did anything but waste time. Some shows can make these scenes work by simply exploring the other characters, but those characters need to be likable and interesting. Agents of SHIELD’s non-Agent Coulson people are getting there, but for now, when the team isn’t advancing the episode’s plot or Clark Gregg isn’t on screen, viewers are more likely looking at their watches than the screen.
When in doubt, go crazier.
People watch ridiculous shows because they’re fun, and the more ridiculous they get, the more fun they are. Whether you’re planning a show — “The Headless Horseman is actually Death!” — or in the middle of it — “The lost colony of Roanoke moved to upstate New York into some kind of weird dimensional pocket and also they have the plague because of Pestilence!” — there’s no downside to going balls out crazy. Again, you can’t break the fundamental rules of the show, but as long as you don’t break your initial deal with the audience, you’re gold. As problematic as Lost’s ending was for some, this is exactly kept people watching for five full seasons without ever practically explaining anything sufficiently. Polar bear! Smoke monster! Hatch! Dharma Initiative! Frozen Donkey Wheel! Temple! And so on and so forth. Whatever you think about Lost now, it was thrilling to watch each new bit of weirdness get revealed, and it kept people watching until the end.