Provo Theatre Company’s production of “Deathtrap” walks the fine line between building suspense and dragging its feet. I fear it may fall too often on the wrong side of that line, but when it’s in gear, it’s a crackerjack of a thriller.

The 1978 play by Ira Levin — which was made into a 1982 film starring Michael Caine and Christopher Reeve — is ABOUT thrillers as much as it IS a thriller, with a witty script that seems to be writing itself as it goes.

Wrap your mind around this: “Deathtrap” is a play about a playwright who comes across a new play called “Deathtrap.” As the play progresses (the play we’re watching, not the play he’s reading), the characters realize that what’s happening to them would, in fact, make for a great play. And so they wind up writing something that closely resembles what we’re watching, except they don’t have an ending for it, because, well, it hasn’t ended yet.

I may have confused you, but the play won’t. Directed by J. Scott Bronson and smartly acted by a tight five-person ensemble, “Deathtrap” falters only in its pacing. The second act is not nearly as thrilling as the first — a fact one character acknowledges — and is way too talky. Tension between the remaining central characters doesn’t build the way it ought to, and so dialogue that might have been ominous now sounds like filler.

Rick Macy is cool and sophisticated as the playwright, Sidney Bruhl, with a great sonorous voice that lends an air of dread to the proceedings. As his wife, Ann Bosler is perhaps too high-strung and fluttery to be believable; their scenes together are off-kilter, like they’re operating on entirely different speeds.

Elwon Bakly plays Clifford Anderson, the clever young playwright with whom Bruhl has an interest in collaborating, and he brings a nice energy to the role. (As a minor criticism, let me observe that Clifford wears sweaters that are far too modern for a play supposedly set before the era of home computers.) Kathryn Little brings comic relief as neighborhood psychic Helga Ten Dorp, and Mark Parmley is suitably repressed as Bruhl’s uptight lawyer.

The play’s best moments are extremely suspenseful and surprising. The theater’s intimate setting that worked so well in “Wait Until Dark” last year succeeds again in bringing the audience into the terror of the situation; surely a vast hall could never communicate such intensity.

But its less-impressive moments unfortunately make up a large chunk of the play, as we drift along, waiting to find out what will happen. It is worth watching, but you’ll be nestled back comfortably in your seat, rather than on the edge of it.