THEY CLOP ALONG with King Arthur and his coconut- bearing knave in "Monty Python and the Holy Grail." They know all the lyrics to the Cat Stevens tunes in "Harold and Maude."

They taunt, "Warriors, come out to play-yay" with the psychotic gang leader in "The Warriors."

And, at the ultimate midnight flick, they dress, dance, walk and talk just like Dr. Frank N. Furter and friends in "The Rocky Horror Picture Show."

Every weekend at the witching hour, the midnight movie- goers come out of the woodwork, ready for revelry. "They're a young crowd basically," says Bob Clinton of the Circle Inner, which has been running midnight movies since the '60s.

Their movies, often "cult films," are usually off-beat, sometimes off-color, and offer more than just entertainment for insomniacs. "It's like going out late for your favorite beer," says Barbara Egan, 23, a midnight movie buff.

First of all, the hour itself is worth a little magic. "It gives people a reason to let their hair down," says Tom McClenahan, manager of the United Artists Movies at Fair Oaks. And at that hour a horror classic like "Dawn of the Dead" can seem a bit more horrifying.

Secondly, more than a dozen theaters in the area cater to midnight maniacs, and many have the sort of atmosphere or the sorts of incentives that keep the crowds coming back. For example, at the Bethesda and Arlington Cinema 'N' Drafthouses, where comedies and thrillers are served up with beer and food, every weekend night is a party.

But perhaps most important are the films themselves. Many have the sort of memorizable scenes worth seeing again and again. They give the audience a chance to participate, to pretend the theater is a clubhouse and they know the password.

In "Monty Python and the Holy Grail" and 'Rocky Horror," for example, well-meaning narrators attempt to "clarify" events for the benefit of the audience. And the audience responds like a classroom full of mischievous kids.

Other movies, like "Harold and Maude," contain moments when the character shares a joke with his viewers.

"You're gonna love this part," a "Harold and Maude" man says to his uninitiated friend, just loudly enough so that others around him realize he's in the know.

The rest of the faithful wait, barely suppressing their laughter, as Harold proceeds to get rid of a computer date. After she thinks she sees him immolate himself in the backyard while she's watching out the window, he strolls inside, with nary a blister nor a hair out of place. As she runs screaming from the room, Harold's mother stares at him suspiciously while he turns, looks directly out of the screen and gives the viewers a devilish grin. They become his delighted, cheering accomplices.

Of course, the quintessential participators are the Rocky Horror-ites. During a wedding scene, they throw rice. When a character covers her head with a newspaper to keep off the rain, they don newspapers. When a toast is made at a dinner party, they . . . no, they don't make a toast, they throw toast, as in toast and jelly. Above all, they know their movie.

"I see you shiver with anticip-p-p-p . . .," teases Dr. Frank N. Furter to his adoring fans in midnightland, a pouting smile on his painted lips.

"Say it!" screams the toast-tossing crowd in unison.

". . . pation."

The timing is perfect.