To call it a mere knife seems somehow insufficient. Insulting, even, conjuring images of dull-edge flatware and kitchen implements. It's more than that: It's The Knife.

Cruelly sculpted and flashing malevolently under the bright stage lights, The Knife very nearly assumes the title role in Anthony Neilson's "Penetrator." In the hands of not-quite-right war vet Stiffy (Jonathon Church), The Knife is the centerpiece of an extended confrontation that leaves audiences breathless, filling Source Theatre's intimate space with tension so thick you could cut it with . . . well . . .

"It does look sharp," says Church with obvious relish. He recalls that when The Knife made its first appearance in rehearsals, "I think we all wanted to make sure it was dull."

Dulled-down, yes. But dull? Not quite. The audience gets to see The Knife in action, readily tearing through a doomed teddy bear, which only ramps up the fear factor when it turns its murderous attentions on its flesh-and-blood castmates.

Still, Peter Wylie, who plays near-penetratee Alan, swears he has never feared for his safety, even during rehearsals. "Everything was spoken through very meticulously and each move -- it's a hokey phrase, but it's a dance," he says. "It's very specific, and it's supposed to elicit a specific response. There was really never any danger." Fellow actor Richard Price, who has his own run-in with The Knife, adds that now that the show is up and running, "It's not a time to be, 'I've got something new I want to work out.' It's set. We're comfortable with it, and we trust each other."

The actors gave The Knife all due respect during rehearsals but worked hard to ensure that the action flowed seamlessly out of Neilson's tautly constructed dialogue and shifting dynamics. Roommates Max (Price) and Alan enjoy a camaraderie that's part "Odd Couple," part "Trainspotting," until Stiffy, Max's childhood friend, appears on their doorstep seeking shelter and muttering about evil "penetrators." His volatile presence sets off a psychological power struggle that ultimately turns physical.

Says Church: "I don't want to say we played a lot in rehearsal with how things were going to happen -- we took it a lot more serious than that -- but we were very open to moving around and making the [stage] picture look interesting all the way up until previews."

Price and Wylie credit Church, a certified fight choreographer, with keeping things safe even though the wacko-with-a-knife action falls outside traditional stage-combat disciplines. "It's not unarmed. It's not rapier and dagger. It's really a different kind of thing," Church explains. "[But] the basic safety practices still apply: eye contact, safe distance, all the early rehearsals at half speed."

The end result is a breathtakingly realistic sequence that the actors clearly delight in bringing to life. "It's hella fun to do, I gotta say," Wylie admits. "I mean, you've seen combat onstage where you're, like, 'This looks like crap.' Jonathon's pretty proficient with that thing, and he moves around and it feels really cool."

"It fits in with the experiential nature of the play -- wanting the audience to experience it all with you," Church says. "If the fight was at all fakey, they would step back. We want to draw them in as much as possible, so the dirtier and more realistic the fight can look, the better."

Jonathon Church threatens Peter Wylie, left, while Richard Price looks on with a smile.