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When Paul Giamatti was offered the role of a sadistic killer in “Shoot ‘Em Up,” he didn’t hesitate to accept it.

“I enjoy action movies, I’m a fan of them, and I’ve always wanted to do one,” he said. “It was a very odd script, kind of unapologetically trashy, which I liked.”

In the film, which opens Friday, Giamatti pursues Clive Owen and Monica Bellucci as they try to shelter a newborn baby from his army of assassins. Owen’s participation in the film was another reason for Giamatti to sign on.

“I thought Clive would be fun to work with,” he said.

Giamatti, who’s appeared in “Sideways,” “American Splendor” and “The Nanny Diaries,” recently chatted about Owen, Karl Rove and playing a killer.

How did you determine that this crazy movie wouldn’t turn out to be schlock?

I don’t think I’d be all that annoyed if it turned out to be schlock. I have a certain appreciation for schlock. I’ve never been too worried about whether I’m going to be taken terribly seriously or not.

Your work in some films is so subtle and this here you’re so pyrotechnical and show-offy. What style of acting is the most satisfying?

Either thing. Whatever it demands is what’s going to be making me happy in the moment. I will tailor what I do, I guess, to the demands of whatever genre it’s in. It felt like I wasn’t supposed to be all that subtle in this. There wasn’t supposed to be a whole lot of shades of gray in this guy.

Director Michael Davis said that you patterned your mad killer after a famous political character on the current scene.

That would be Karl Rove. I remember thinking what would be interesting was to make him the most unimpressive-looking guy who happens to be incredibly powerful and that was who came to my mind. But I wasn’t trying to do an imitation of him.

There’s a naughty line in the script that directly refers to your best-known film, “Sideways.” How did that come into the film?

Michael suggested it and in actual fact I didn’t get it until about a week later. And I said, “Yeah, sure, I’ll say that.” Then later I said, “Oh, I see why he wanted me to say that.” I didn’t get it, but I’m glad others do.

What was the most demanding sequence to shoot?

Clive really did most of the hard stuff. I kind of stand there and shoot. It was tricky wrangling that big, huge pistol. A pain in the neck, that thing. The barrel’s got to be a foot and a half long. It was difficult to run around with that thing and whip it out in an impressive way. But the scene where we’re charging up the stairs, shooting at him, we did about 17 times. And that actually was a bit of a pain.

The confrontation scenes are so preposterously intense. Did you and Owen have a tough time not cracking up?

We cracked up through the whole thing. The scene where I was breaking his fingers, we couldn’t stop laughing. Because there’s these ridiculous prosthetic hands that I was breaking. It was very funny to do.