While actor David Oyelowo might be best known to some audiences for his stellar performance as Martin Luther King Jr. in the epic civil rights drama “Selma,” by the time he filmed his breakthrough role, he’d already completed work on “Nightingale.” The movie, which stars only him and premieres on HBO this Friday at 9 p.m., tells the story of the deteriorating mental state of a vet named Peter Snowden as he tries to keep his life together after committing a very brutal act. Oyelowo is virtually unrecognizable as the same performer who played Dr. King – the only parallel between the movies is that they’re both produced by Brad Pitt’s production company, Plan B. We talked to him about being your own co-star and working with Pitt.

The film occasionally feels almost like a play, in the sense that it’s a one man show. Did that have any impact on how you made it?

I did learn it like a great big monologue. I didn’t do what you may otherwise do, which is that you learn your lines for any given scene a couple of days before you actually shoot it. I felt that the level of immersion that I would want to subject myself to in playing this character would mean that I didn’t really want to be in the head space of technically learning lines whilst shooting, so I did learn like you would a play. But then the rest of the experience was that of a filmmaking one in terms of the nature of the way we shot the film. Because it’s very intimate in the way that theater can’t be. He’s expressing very dark thoughts at times, right up close into the camera, and that employs a very different part of your artillery as an actor than the stage does.

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Did you find yourself recalibrating your performance at all, since it’s just you onscreen?

I don’t feel like I did recalibrate what I would normally do as an actor. It was just doing what felt right and true for playing this particular role. I don’t think we did anything that was technically different, per se, than what you would do in any other given film, but because of who he is, the way he expresses himself, the fact that this is a guy who will happily have a conversation with a three way mirror, means that the realms of the orthodox ways of either shooting a film or interacting with another character are kind of out of the window, so you’re just doing whatever feels truthful given Peter Snowden’s circumstances.

Peter spends a lot of time on the phone, either talking to people, or rehearsing talking to people. Was someone on the phone with you at those times?

We actually made the choice to never have anyone on the other end of the line. It was something I felt very strongly about because there are times with Peter where there is someone on the other end of the line and there are times when there isn’t. It is someone he has manufactured in order for him to live with himself. The film starts with Peter having committed a heinous act and he has a mental condition that means that dealing with the repercussions of both what he has done and who he is is something that he is constantly pushing away…. We wanted to keep the audience guessing as to whether he’s actually talking to someone other than himself.

This is the second time you’ve worked with Brad Pitt, who also produced “Selma.” Do you think you’re drawn to similar types of projects?

It’s funny, because we’ve now had two projects together, but actually my friendship with him is very much linked to us being dads with lots of children. He has six, I have four, and we get together every now and again and watch our children tear the place up. But yes, we worked together on “Selma,” and “Nightingale” was born out of that. While I was doing “Selma,” we had already shot “Nightingale” and Dede Gardner and Jeremy Kleiner, who both run Brad’s company Plan B, I showed them the film, which we had finished at that time. … They saw it, showed it to Brad, and I remember him saying, my goodness, what a thing to go from Dr. King to Peter Snowden. His connection with “Nightingale” was very much from an actor’s point of view. I think he was very interested in an actor putting themself through that exercise.