* Deathtrap has been running on Broadway for ages. In fact, I saw it so long ago that I couldn't remember all the story twists when I previewed the movie version the other day. And that was all to the good, because twists are what this hugely tricky thriller is all about: Virtually nothing is what it seems, and every action has an equal - and sometimes violent - reaction, in precisely the opposite direction.
As directed by Sidney Lumet, the screen ''Deathtrap'' (rated PG) is more explicit than its stage counterpart, in its occasional violence and homosexual undertones. Its performances aren't exactly understated, either, as Michael Caine storms around the house while Dyan Cannon punctuates the soundtrack with unearthly shrieks, and Irene Worth pours on what might be the thickest Dutch accent in movie history.
But the story is a rattling good one, as they say, with its stagy skullduggery among a failing playwright, his nervous wife, an eager protege, and a ''psychic'' from down the street, all snagged in a goofy plot where it isn't even certain who'll wind up murdered. Christopher Reeve, in a surprisingly vulnerable mood, is just right as what used to be called the ''juvenile,'' and the Long Island setting is as expressive as it is attractive. ''Deathtrap'' falls short of the classic potential it would obviously like to have. Still, it's a jaunty entertainment, by and large.
* Film has become an important tool of ''experimental'' theater. Recent productions by a number of leading Off-Off-Broadway troupes have prominently featured movie sequences. To name a few: ''Route 1 & 9'' by the Wooster Group, ''Wrong Guys'' by Mabou Mines, ''Specimen Days'' by Meredith Monk, and ''A.M./A.M. -- The Articulated Man'' by the Fiji Theater Company. Meanwhile, other avant-garde gurus such as Richard Foreman and Charles Ludlam have turned their talents to filmmaking as well as dramaturgy.
Perhaps the longest-running of all such ventures this season has been Mr. Dead & Mrs. Free by the adventurous Squat Theater, a group of European expatriates who have been ensconced in New York for some years now. The show is about people, including a policeman and his wife, who live out various contemporary problems, from compulsive consumerism to violence. Just like a Hollywood movie, it begins with filmed credits, and continues on film for about 30 minutes before the first live performer appears.
The production contains other gimmicks, too. The stage is dominated by a huge dummy with TV sets instead of eyes. Music and sound effects flood the theater. A friendly dog mingles with the audience. The sidewalk outside the building, visible through a plate-glass wall, becomes part of the scenery -- and provides an unusual dimension to theatergoing, as passersby become unwitting parts of the show.
Some of these maneuvers work very well, lending necessary punch to the play's rather dated metaphors (mainly the Vietnam war) for contemporary decadence and despair. And the movie portions help broaden the action beyond the limitations of the troupe's obviously low budget. But it's ironic that the very seamlessness of the filmed and ''live'' portions will probably end the usefulness of the ''Mr. Dead & Mrs. Free'' movie when the stage production closes itsdoors. It would be interesting to see what the Squat troupe might come up with if they attempted a film for its own sake, without anchoring it to a complicated theatrical base.