You know him best as Bill, the bodacious teenage time traveler in Bill & Ted's Excellent Adventure. But now Alex Winter is trying out a different role: He's cowriting, codirecting, and costarring in MTV's new after-hours comedy series, The Idiot Box (Saturdays, 11-11:30 p.m.; repeated Mondays, 12 midnight-12:30 a.m.). ''It's sort of an American version of Monty Python,'' the 25-year-old actor says. ''It's a fast-paced, violent, cathartic style of humor. It's perfect for MTV.'' Says cowriter-codirector-costar Tom Stern: ''It's a completely different approach to comedy. It's kind of an experiment in ultra- slapstick.'' So far the duo has created six 12-minute installments (about five sketches per episode, with music videos shown between every skit), introducing such off-kilter characters as Eddie the Flying Gimp from Outer Space and Stan Harding, Rock Accountant, a number-crunching geek who specializes in heavy metal bands (''By depreciating the flashpots up front, we reclassified the entire light show as rolling stock and not taxable assets! Wheeew! Is that wild or what?''). Does this new project mean Winter has given up time travel? Not a chance. He's been on the set of the sequel to Bill & Ted's Excellent Adventure since January; the film will arrive in theaters this summer.