Sundance wouldn't be Sundance without an appearance or four by Sam Rockwell, whose superb sci-fi effort Moon features the actor playing opposite one of his most formidable co-stars to date: Himself.

We'd say spoilers follow, but it's hard to say whether the reveal that occurs at Moon's half-hour point is in fact a twist or just a requisite plot point ushering in the film's second act. Either way, one can't describe Rockwell's performance without acknowledging his dual portrayal of Sam 1 and Sam 2, each an unaware clone of the original Sam Bell, an astronaut "harvesting" energy on the moon. When Sam 1, mere days from returning to Earth, experiences an accident and the second Sam is brought to life by his base's friendlier, Zolofted version of 2001: A Space Odyssey's HAL 9000 (voiced by Kevin Spacey), a breach in protocol brings the clones face-to-face as they settle a mutual, potentially deadly resolution.

To loyal Rockwell fans, Moon may not immediately reflect his best work; it's jittery, quirky, too conversational. But as his story uncoils — as he fights himself, challenges himself to ping-pong, and grapples with his nature folding repeatedly onto itself — the seams disappear and the accomplishment seems to defy such easy reckoning as "best" or "worst." It's simply another marvel we'll file alongside his others, even as we wonder how the hell he did it. Rockwell and Moon director Duncan Jones sat down with Defamer this afternoon to attempt an explanation.

DEFAMER: You know, we checked this out last Friday and still don't know exactly how to write about it. The twist comes in the first act, and that's just the start. How did you hammer out the concept?

ROCKWELL: Well, it explores loneliness. That's a vague enough thing. We can say that.

JONES: We can say that. It was written for Sam. We basically had a meeting and seemed to get on pretty well. Both of us are only [children], but we had sort of this fraternal thing.

SR: We geeked out on films. Sci-fi films in particular, but a lot of other films.

DJ: The stuff when we were growing up — Outland, Silent Running, Alien, just these films that had this grittiness and clunkiness. No "clunky" in story, but everything felt hard and real and blue-collar. Actually, Sam, I remember that was one of the things you talked about; wanting to play a blue-collar guy. So we went away from that meeting and it was already in my head that I wanted to write something for Sam, and it needed to be something that appealed to him but appealed to me as well.

D: But you’re a very technical actor, too. How did you get your head around having to act opposite yourself?

SR: This is the most challenging technical film I've eve done, except maybe Hitchhiker's Guide to the Galaxy. But that was just prosthetics. This was so crazy. We had this motion-control camera—

DJ: It’s not the easiest thing to concentrate.

SR: It was the loudest camera.

DJ: It’s based on a system of rails, and the movements can be replicated exactly. So you can create layers on of visuals and put them directly on top of each other and everything will be in sync. We can have multiple Sams!

SR: And we were sort of terrified in the beginning whether it would work or not, creatively or technically. And when it did work — when we saw our first shot of the two guys together? We were so ecstatic.

DJ: It was a milestone. The whole film depended on this effect working. And at an indie budget, you can’t do loads of technical tests beforehand. And make sure everything is going to work exactly the way you want it for each shot.

SR: They had three times the amount of time we had on Dead Ringers or Adaptation to do the same effect. The only thing they didn't have was an iPod, which I watched previous takes on.

DJ: We studied the Criterion version of Dead Ringers, where they explained how they did it. It was really helpful.

D: You mentioned your other influences a moment ago. Some critics have said Moon is too overtly derivative of films like 2001: A Space Odyssey or Solaris or Alien. How did you perceive your relationship to those movies when you were making this?

DJ: I can honestly say that 2001 was not a direct inspiration for this film, because our inspiration was the films that were inspired by 2001. We're second-generation 2001.

SR: It's funny. I went back and watched 2001 and the making of, and then I realized that Blade Runner and Outland and Alien had all been influenced by 2001. So everything is derivative. But it's great because you're paying homage to your forefathers.

D: When we think of sci-fi at Sundance, it usually means high-concept — Primer, Pi or Sleep Dealer last year — as opposed to production design or effects like what you have in Moon. How did you accomplish this on a low budget?

DJ: It was like military strategy. We went more retro; we went with modeled miniatures like the original Alien — a lot of things that you wouldn't do in-camera these days. We chose effects that we knew would would be most cost-effective and could reuse over the course of the film. We really wanted more time for Sam. We had to do these really technical shots that were one take.

SR: It was unbelievable. And all the more triumphant when we saw they worked. Like the ping-pong scene?

DJ: We did three or four takes of the first Sam, and got something the second Sam could rehearse to for one take. But the problem is because there's a table-tennis table there, and it moves

SR: Remember I moved it once? And you almost had a heart attack? I think I moved the net or something.

DJ: And I disappeared with the special-effects advisor and had a conference to determine how we could split the two Sams and make sure the table was in the right position. Making effects work on the day was something we had to do a lot of.

SR: I came to them with a scene from Midnight Cowboy where Joe Buck and Ratzo Rizzo embrace each other. And I said, "Can we do this?" And the thing you have to understand is that to have the two clones touch each other is such hard work technically, it's almost impossible. But they came up with a scenario; they said, you can't do that, but you can do this. And it's amazing.

D: But what about the mindfuck of it all? Beyond the technique, you're thinking and reacting on the spot to, like, the other you.

SR: It was crazy. I was losing my mind sometimes. I think we both were. It was surreal. For instance, we'd rehearse one of the clones, and then the assistant director said, "Get into make-up." The first clone had extensive make-up that took about an hour. I'd say, "Wait a minute — I have to rehearse the other clone, otherwise I'll have no idea where I'll be." It was very strange. You've got an earwig, you're looking at a tennis ball and acting with the previous take of your own voice.

DJ: It's a one-man show, but everything is out of order. So Sam's trying to maintain the arc of the character while breaking it up and then doing another character's arc as well.

SR: It was fucking nuts. It was terrifying.

DJ: But the one thing everyone can agree on is that it looks real. I think the results speak for themselves.