NEW YORK — Like his star sign, Jason Butler Harner is a Libra "of epic proportions." That means he always seems to be weighing things.

"I'm a constant waffler," the actor says. "I've said to anyone I've ever dated, 'Asking me to choose a restaurant is going to be a conversation that's going on for a while.'"

How perfect then that Harner's latest role is in a play about having to choose — or the agony of being unable to choose. Except that in the case of Mike Bartlett's clever British import "Cock," the choosing is one's own sexuality, not between dim sum or curry.

The play centers on a young man torn between his long-term male partner and a young woman he has accidentally fallen in love with. The young man simply cannot make a decision, which leads to plenty of spats and a dreadful dinner showdown with all three parties and one of their fathers.

"A friend said it's like reliving every breakup you've caused or endured or counseled on or suffered through," says Harner. "The older you get in the audience, the more you probably have experience with this."

Critics have praised Harner, who gives an anguished, funny and touching performance as the horrified boyfriend who now must fight a complete stranger for his lover's heart. The rest of the cast includes Amanda Quaid, Cory Michael Smith and Cotter Smith.

"I needed a skillful actor who could do both the width of it and the edge of it, but also make the guy sympathetic and human and understandable. Jason absolutely does that," says director James Macdonald. "He's been amazing. He's technically amazing but emotional amazing as well."

Much of the power of the play, which made its debut in 2009 at London's Royal Court Theatre, is in its spare, coiled language. There are no props, just actors circling each other like wary roosters in a cockfight, slashing each other with their verbal claws. Hence the rather shocking title.

"You don't seem to have grown coherently," Harner's character taunts his lover at one point. "You're a collection of things that don't amount. You're a sprawl. A mob. You don't add up."

The set is made up of five rows of raw wooden benches arrayed concentrically around a tight gladiator ring, giving the 120 or so watchers at The Duke on 42nd Street an uncommon and arresting view that sometimes feels like participation.

"It's interesting. It changes vastly every night," says Harner, 41, who was last seen in the now-canceled "Alcatraz" on Fox. "The more bloodthirsty the audience is, the more fun it is."

Sitting in the empty, wooden auditorium, Harner confesses his other skill: carpentry. "I love it," says the man who has a cordless drill under his bed and has been known to build fellow actors a chicken coop. "In theater acting, you leave that night and what it was is what it was. But when make something, there's this thing I made."

The play offered Harner, a self-identified "journeyman actor," a chance to return to the stage after the TV series and reinvent himself. He at first had found himself playing a lot of emasculated, somewhat effete men in works such as "The Glass Menagerie," ''Long Day's Journey Into Night," ''Hedda Gabler" and even Ivan Turgenev in Tom Stoppard's masterful "The Coast of Utopia."

So Harner, a graduate of New York University's Tisch School of the Arts, felt he needed a little more danger in his roles. Along came the part of an unnerving, creepy killer in 2008's "Changeling" opposite Angelina Jolie. Soon he was being cast as the bad guy — a new trap.

"I miss getting to be funny. I miss language and I love language so much. This was a no-brainer. And the no-brainer choices are rare. I was looking for a play that reminded me that I am an actor," he says.

"I am blessed to have made my career as an actor for a decade now and I'm grateful for that. It could arguably have gone better in ways and it certainly could have gone worse, but you have to keep fueling the fire."

Bartlett's play has been personally intriguing as well for Harner, who has struggled with his own sexual identity and has had feelings for both men and women at different times. He watched his father come out of the closet at 52 after two marriages.

"This play has been fascinating," he says.

Harner says he's in a fantastic relationship with a man — a writer and journalist, with whom he is apartment shopping — but he's learning all the time about commitment and how to make a relationship work. And about being decisive.

"As a kid, I was always like, 'The world says everything is black and white, but there's a world of gray,'" he says. "Then, as I'm getting older, I'm like, 'Gray is very important. But sometimes you just have to make a decision and you have to go with it.'"

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