"I was tricking him all the time," John C. Reilly says of Criminal co-star Diego Luna. "He’s a lot of fun to be with and he’s really gullible. I’d call him on his cell phone in the middle of a take. I’d have my hand in my pocket, his number programmed in, and I’d hit ‘Send.’ He’d have his personal cell phone on him, on vibrate, and in the middle of a take you’d all of a sudden see him go – " Reilly says, doing a horrified Diego Luna face.

But as you may have heard, or read, or seen watching Chicago, Reilly is a really nice guy. All this chicanery was done, really, just to get into character.

"A shark," Reilly says of Richard Gaddis, his character in Criminal, out Sept. 10. "Someone who basically makes money on other people’s weaknesses. He feels that the normal rules of life don’t apply to him and sees life as a series of opportunities. He’s the ultimate opportunist."

"It’s invigorating to play a character who doesn’t have the normal worries of everyday life that you and I have — like moral issues or ethics," Reilly adds. And with the aid of his new partner Rodrigo, played by Luna ("We gotta Anglo you up," Richard tells him in the film), the two enter into a game of con.

For the real life Reilly, however, cons just included harmless pranks such as inappropriately timed cell phone calls and seat warmers on hot days for his targeted recipient, co-star Luna.

"Our Mercedes had seat warmers — it must have been imported from the East Coast," Reilly surmises. "And if I did it once with the seat warmers, I must have done it 50 times! It was really warm in the car and we couldn’t run the air conditioning because we were filming. Diego would start sweating and saying, ‘Man, it’s hot in this car!’ And I’d joke, ‘I thought you were from Mexico, aren’t you used to this heat?’ Then he’d realize I’d done it again, screaming, ‘Dammit! You got me again!’ He started putting tape over the buttons so I couldn’t do it."

Reilly says that it wasn’t just the possibility of showing the movie-going audience that he could play a jerk that attracted him to the role. "What sets this film apart from other con movies is that smack in the middle of it is the issue of Richard’s dysfunctional family," he says.

"Once he starts dealing with his sister (Maggie Gyllenhaal), the story rises to a different level. They’re fighting over their mother’s estate and he’s been pulling strings to cheat her and their younger brother out of their shares. The stakes in this story are much higher for Richard than simply the money from the scam. Is Richard just a hard-bitten tough guy?

"I love that in the movie you are constantly guessing people’s motivations and intentions," he continues. "And I think audiences will be able to relate to some of the issues between the siblings."

For Reilly, however, one of the most satisfying parts of being in Criminal is that, after some 35 films, this is the Oscar nominee’s first leading role.

"It’s going to sound ungrateful if I say, ‘Oh, all this time I’ve wanted to do leads.’ But I’m an actor! That’s what I love to do. The more I get to do it, the happier I am. The more time I get to spend on set and less in the trailer, the happier I am every day. You’re more tired at the end of the day, but it’s a good sort of tired.

"One of the producers of Chicago said to me, ‘Potential is a terrible thing to have. It will corrupt you inside if you don’t get rid of it.’ Of course I want to do more," he says of acting. "It’s what I do for a living."

Criminal releases in theaters Sept. 10.