Charlie Day is good at playing a fool

The first scene Charlie Day shot with his costar Jennifer Aniston on the set of the new comedy “Horrible Bosses” required him to sprawl out in a dentist’s chair as though he had been drugged while the tanned and taut actress, dressed in lingerie, straddled him predatorily.

“It was awkward,” Day said, recalling the scene months later during an interview. Then again, the 35-year-old added pragmatically: “Actors put ourselves in awkward positions all the time. There’s something methodical about it. You stand on a piece of green tape and say a line or you stand on a piece of green tape and pretend you’re passed out while someone’s half-naked on top of you. If you can’t pull that off, God help you.”

In the Warner Bros. film opening Friday, Day plays Dale, a dental hygienist who is about to be married and must endure the unwanted sexual advances and bawdy banter of Aniston’s aggressive Dr. Julia. He is the most naive of three friends who conspire to murder their abusive bosses — Jason Sudeikis plays an accountant for a corrupt, coke-snorting creep (Colin Farrell) and Jason Bateman a middle manager for a sadistic control freak (Kevin Spacey).

“People tease me about the part, like, ‘Oh, I wish I got to play that.’ But I definitely identify with the character,” Day said. “If I were in that predicament I’d be extremely uncomfortable and I wouldn’t enjoy work at all. I didn’t want to totally deny that Jennifer Aniston is attractive or sexual or that it would be a struggle for the character, but at the same time, there’s no scenario in which the week before your wedding you do something like that [cheat on your fiancée], cause it would be way worse for you.”

Day brings some unconventional assets to the role that help sell the idea of him as a man-eater’s ideal victim. He’s small enough for the 5-foot-6 Aniston to tower over him menacingly in stilettos — and has a scratchy voice pitched just south of Bobcat Goldthwait when he’s excited.

“Knowing that I’m not a model and I’m never going to be has relieved me of the pressure of looking good,” Day said of the advantage of his bearded every guy looks. “If you don’t establish yourself as McDreamy then you don’t have to live up to it.”

His distinctive voice, however, is a little more of a mixed blessing. “Starting out, I bet I didn’t get a lot of parts because of my strange voice. I’m not consciously thinking, ‘hey, sound like a squeaky dog toy mixed with a bagful of rusty nails.’ It’s just what my voice has done.”

Day had just flown into L.A. from shooting the seventh season of his slacker sitcom, F/X’s “It’s Always Sunny in Philadelphia,” and was downing black coffee before heading to a news conference to promote “Horrible Bosses,” his first leading role in a film.

Dale isn’t exactly the odd man out on Day’s résumé; the actor has made a career out of playing idiots and oddballs. His early TV and film work included parts like “uncredited stoner,” “mailroom kid” and “inbred twin.” His is the dimmest character among a cadre of slow thinkers at a Philadelphia dive bar on “Sunny,” and in 2010’s “Going the Distance,” Day played a sweeter variation on the moron theme — a hopeless romantic who supplies the mood music for his friend’s hookup.

“We talked a lot in rehearsal about how dumb should we make these characters,” Bateman said. “Charlie plays such a great simpleton. You need to have a lack of ego to do that. Sometimes actors put on their suit of armor to protect themselves and it’s a shame. Then it becomes, ‘Well if this guy’s not going to pull his pants down then I’m not going to pull mine down.’ You don’t have to have to ask Charlie twice to unbuckle.”

It was a combination of Day’s on-screen commitment to unbridled absurdity and his off-screen commitment to his wife, the actress Mary Elizabeth Ellis, that won him the role, according to director Seth Gordon.

“The guy had to plausibly be so smitten with his fiancée that you believe that Dr. Julia’s advances are a problem,” Gordon said. “That’s the truth of Charlie’s life. He’s got a lovely wife and they have a very sweet relationship.”

Day’s own roots are much more cerebral than the rubes he plays. He grew up in Middletown, R.I., the child of two music teachers, and is the only person in his nuclear family without a doctorate degree. He plays piano and guitar, and writes some of the music on “Sunny.”

“My parents are more likely to know who Franz Liszt is than Snooki,” he said. “We didn’t have a car radio when I was a kid, which I always thought was weird for two music teachers. They opted for the cheaper model.”

He majored in art history at Merrimack College in Massachusetts and after graduation pursued acting at the Williamstown Theatre Festival. Performing a jokey blues song he’d written at one of the summer stock’s cabaret nights earned Day his first agent, and he began working in New York in theater and small television roles.

He supplemented his sporadic income with a series of odd jobs — busing tables and answering phones for a telethon selling a Motown anthology.

In 2005, a home video Day shot with his friends Rob McElhenney and Glenn Howerton for less than $50 evolved into the pilot for “It’s Always Sunny in Philadelphia,” which was picked up by FX and has gone into syndication on Comedy Central. One of the show’s writers and executive producers, Day, who plays a character named Charlie, has matured as a performer during its making, according to Danny DeVito, who joined the series in its second season.

“You can feel people settling into their skin,” said DeVito, who plays Charlie’s roommate and suspected biological father. “They were all very green, including Charlie. He’s much more comfortable on the stage now.”

The show also brought together Day and Ellis, who live in Silver Lake — she plays a waitress whom Day’s subliterate barkeep Charlie lusts after. They met as fellow theater actors in New York City in 2001 — “We just hit it off and I’ve spoken to her every day since,” he said — and are expecting their first child in December.

With the new season of “Sunny” wrapped, Day will next tackle his first role in a studio tentpole film, Guillermo del Toro’s monster movie, “Pacific Rim,” which will afford the actor new creative terrain. “I play a scientist. This guy can read and write,” Day says. “Unlike my other characters, this guy’s not an idiot.”

Reflecting on his summer stock days, Day said he always assumed he’d find work as an actor, if on a smaller stage.

“Thinking of Plan B muddies up your chances of succeeding at Plan A,” he said. “As soon as I got my foot in the door I’ve been able to pay the bills. It did not occur to me that that might not happen. Maybe if you come from a lot of wealth you worry about not having it, but if you’re comfortable living in a ratty apartment, you don’t worry about going back to it. Although now I’d be worried about going back to it. No more tiny apartments in New York.”

rebecca.keegan@latimes.com