"Oh, yes, it's uncomfortable," Reilly says. "It just is. Like, how do you broker that situation? When you're dating someone who has little children, you can shelter them. But when someone's 21 years old, there's got to be that moment where they're looking across the breakfast table at each other, the kid and the new boyfriend, and it's like, 'You are having sex with my mother and that is not right!' "
Directed by Jay and Mark Duplass - sibling perpetrators of the highly esteemed, low-budgeted The Puffy Chair and the mumblecore horror pic Baghead - the decidedly edgy Cyrus was dreamed up with Reilly in mind. The film opened Friday at the Ritz Five and Rave Motion Pictures Ritz Center.
"Jay and Mark wrote this script, and we met and they said, 'Well, we hope you want to play this character, because if you don't, we're not going to make this movie,' " recalls Reilly, in Philadelphia recently. "'It's not like we can replace you with someone else - you're the only person we're thinking of.'
"And that was like, Whoa! All right. I guess I better do it then."
A serious actor with a seriously distinctive mug, Reilly has been showing up in movies for more than 20 years now, since his screen debut as an Army private involved in the gang rape of a Vietnamese girl in Brian De Palma's Casualties of War. He played a porn actor in Paul Thomas Anderson's Boogie Nights (and has appeared in Anderson's Hard Eight and Magnolia), was Renée Zellweger's husband in Chicago (a role that landed him an Oscar nomination), and had the title role in the music-legend biopic parody Walk Hard: The Dewey Cox Story, which he also helped to write. He's worked for Woody Allen and Terrence Malick, Martin Scorsese and Robert Altman, and twice teamed with Will Ferrell, in the hit knuckleheaders Talladega Nights and Step Brothers.
Reilly, 45, hails from Chicago, the fifth of six kids. There was a great uncle in vaudeville, he says, but apart from that no one among his extended Irish-Lithuanian clan even dabbled in show business, or theater, or any of the arts.
"I had no reference points for that as a kid," he says. "I come from a neighborhood where that's not really done, but I've always seen myself as an actor, it's what I've always done from the time that I was a little kid."
His acting epiphany: Writhing around on the floor, pretending to be a piece of bacon "slowly cooking in a pan" at a drama camp his friend talked him into attending. Reilly was 8.
"I remember thinking, I could do this all day. I could be a piece of bacon all day," he says. "And then I just never stopped."
Still, it wasn't until he was in the theater program at DePaul University that the idea of making it a profession seemed even remotely possible.
"I was almost done with school, and a friend of mine who was from the same neighborhood as me got cast in Peggy Sue Got Married, that Francis Ford Coppola movie," Reilly says. "And I realized then that it could happen to someone like me. From that moment, I was like, I should try to make this my job. . . .
"But my dad was just amused by the whole thing. He kept insisting that I take business classes in college, just to be prepared . . .
"He came around when I did my first movie job and he saw how much money I made. He was like, 'Yeah, I think this could be a good thing, John, stick with it, I always knew you could do it.' "
Reilly says that his father, despite his early lack of faith in his son, was a huge movie fan.
"He loved old movies, and was an expert on character actors - Walter Brennan, Sydney Greenstreet. He'd watch old Bogart movies and he could name all of the character actors. And it's funny, because people so identify me as this character actor now, and kind of put me in that lineage, and my dad worshipped those people. He didn't really live long enough to see me fully come to fruition. I'd done probably 10 movies or something when he died. But he was very proud. He couldn't believe the success that I had had."
Reilly lives in Los Angeles with his wife, Alison Dickey, an independent producer, and their kids. In addition to Cyrus, he has two titles in the can: Cedar Rapids, a comedy set in the world of insurance sales, with Anne Heche, Ed Helms, and Sigourney Weaver (from Reilly's The Good Girl director, Miguel Arteta), and We Need to Talk About Kevin, "a very sad, somber movie about what it's like to raise a kid that doesn't like you" - and who goes off and commits a horrible crime. Reilly is the kid's dad, Tilda Swinton the boy's mom.
There are other projects in the works, Reilly says, and there's even talk of a Step Brothers sequel ("it's become this real cult movie - people love it, quote it"), but nothing that's certain at this point.
But he's glad to talk about Cyrus.
"A cool thing about this movie is that a lot of the characters do unsympathetic things, or weird things, or overly needy things, but it all comes from a place of love," he says. "It's not because they're bad people, or they're deliberately trying to be bad. They're just desperate. Everyone has moments of desperation."
Contact movie critic Steven Rea at 215-854-5629 or email@example.com. Read his blog, "On Movies Online," at http://www.philly.com/philly/blogs/onmovies/