“Like any powerful work, it can be read on any number of levels,” William Friedkin said. The legendary director of “The Exorcist” and “The French Connection” was in Dallas talking about “Bug,” the play by Tracy Letts he’d adapted for the screen starring Ashley Judd and Michael Shannon as a couple in an isolated motel room who become obsessed with the thought that they’re infected with tiny insects burrowing beneath their skin—the result, they believe, of some horrible experiment aimed at controlling them.

“It’s really about how, when people are that lonely and isolated, and meet someone who’s on the same page with them, they tend to adopt the paranoia of the other person,” Friedkin added. “And the separate reality that all of us live in.”

Elaborating on the last phrase, he added, “I don’t know whether all of this is in their heads or not. I can tell you this—most of my life is in my mind’s eye. I will confess that to you, that I live in my own reality, and I don’t know how other people see the world. I really don’t. I often find that my wife and some of my closest friends see the world differently. That’s why we go to movies and read newspapers and go on the Internet—to prove that there are other people there, to prove that we’re not all just a dream in the mind of God. I find that as I get older, I keep to myself more and live inside my head more.”

When Friedkin first encountered “Bug” on stage, he was immediately drawn to the idea of filming it, especially with Shannon, who originated the role of Peter, the strangely withdrawn young man who infects Agnes (Judd), the troubled waitress who befriends him, with his belief in the insects living under his skin. But he had competition for the rights. “There were some major stars who wanted to play that part,” he recalled. “A couple of them tried to buy the play for themselves. But I thought Shannon was born to play that part. I couldn’t think of anyone any better. This was written for him.” He also remembered what his co-star had said at the wrap party: “On the day we finished, Ashley gave a little speech to the crew and said, ‘I never worked with Marlon Brando or James Dean, but working with Michael must be what it was like to work with really great talents like that.’”

Judd hadn’t played Agnes on stage, but Friedkin had immediately thought of her for the role. “I always thought she was better than a lot of the parts she was getting, and capable of more,” he said. “There aren’t a lot of roles like this one; let’s face it, it’s very challenging.” Friedkin added that he always looked for intelligence first in choosing a cast, “and I found that in her. She’s breathtakingly beautiful—her skin would have turned Da Vinci on—and she’s very charming and open, but really smart, and a very complex person. She’s very warm, open and at the same time fragile. And the minute I saw this piece, I thought about her and Shannon.”

Friedkin explained that he had very definite ideas about how to approach “Bug.” He shot it almost entirely on a set built in a high-school gym in Louisiana—“except for the exteriors, we did them all at the end”—and totally in sequence. The exterior shots were additions to the one-room setting of the play, but he said, “I wouldn’t say that I opened it up. But there are outside shots, because I wanted to emphasize the reality of this piece. I wanted to set it in a real area, a real place, as remote as it was. I felt that the audience should have some reference to the fact that it was a real motel on a deserted highway, with other cars around there, that it wasn’t just set in a kind of limbo. And I thought I could still maintain the claustrophobia.”

Then Friedkin laid out every shot before filming. “I’ve never done that before,” he said. I drew out every shot. I always see a film in my mind’s eye, and then I’ll tell the crew what we’re going to shoot the night before, but I don’t have any drawings or anything—I’ll just say we’re going to go here, then we’ll move over here and do this. I’ve never drawn anything, not even any of the chase scenes I’ve done. I’d just have them in my head and talked to the crew about what we’re going to need to bring them off. But [in this] I drew every frame and varied it only slightly, because I wanted to make the film fast and low budget. So we shot in twenty days, and the film was made for under $4 million.”

Nor did he rehearse before the shoot. It was a lesson he learned with “The Exorcist,” on which he’d rehearsed the cast so intensely that their performances had become too calculated by the time filming started, and he had to tell them to scrap what they’d done and start anew. “You have to discover these people on the spot,” he remarked about the actors’ relation to their characters. “You can’t do it like you’ve read the script. And I’ve never rehearsed since, for that reason.

“What I do is, I’ll talk to the actors. We’ll talk at length for weeks before we go anywhere to start shooting, about who they are and what they’re doing there—what’s real and what isn’t, if that can be determined. And then I’ll give them some idea of how I’m going to stage it.”

And he added, “I don’t like to do more than one take anymore, either.”

The mode of filming—economical, spontaneous yet precise—combine with Friedkin’s careful selection of actors to explain the intensity and visceral power of “Bug,” a Lionsgate release.