Caissie Levy and Will Swenson were so used to playing lovers that they weren't sure how to play enemies.

They first got romantic in the 2009 revival of Hair, as the doomed couple Sheila and Berger. Then last year, they cast forbidden sparks as the adulterous leads in the rock musical Murder Ballad.

But for their latest pairing, they're playing Fantine and Javert in the new Broadway revival of Les Misérables, which opened on Sunday at the Imperial Theatre. In their only major scene together, Fantine, who has recently been forced into prostitution, fights an abusive john — so Javert, a police captain, arrests her. It's anything but a sexy moment.

"In the first few passes through the scene, as I was looking down at her, I was saying, 'Why isn't this jelling? Why do I feel such a different vibe on stage with Caissie?'" Swenson recalls. "And it dawned on me that this was the first time I hadn't had a sexual dynamic with Caissie in a show. We've always just made out the whole night long."

That's not a typical problem for actors. As they move from show to show, they're often part of a changing constellation of co-stars, rarely appearing with the same people twice — let alone three times in quick succession.

But despite that moment of romance confusion, Levy says she's glad to work with a familiar face.

"On the first day of rehearsals — going into a high-pressure situation with so many expectations and so much excitement — looking across the room and seeing someone who's known you for five years is incredibly comforting," she says.

Plus, since Javert and Fantine don't interact that much, one actor can study the other's performance.

"Throughout rehearsal, there was a lot of checking in," Levy says. "'How does this moment feel? Does this look false? How is this reading?' Everyone has crutches that they lean on without always realizing it, and it's really valuable to have someone you can check in with to say, 'Am I pushing myself hard enough here?'"

Frequent pairings aren't just good for the actors, of course. Plenty of audiences would love to see a new Mandy Patinkin-Patti LuPone match-up, or watch Nathan Lane and Matthew Broderick tackle another comedy.

Still, reunions can come with a cost. Tara Rubin, the casting director who assembled the Les Misérablesrevival, always thinks about prior history when she's calling people for auditions.

"Sometimes, I feel attracted to the wealth of possibilities because actors have that rapport," she says. "But sometimes I think, 'Well, we've already seen this couple together.' Or, 'We've already seen these guys in confrontation. Would it be more interesting to match this actor up with someone else?'"

No matter who has worked with whom, Rubin continues, "the first responsibility of everybody who's on the team is to tell the story."

'The Door Is Already Open'

The stars of The Bridges of Madison Countywould argue that the story they're telling benefits directly from their familiarity. In the musical adaptation of Robert James Waller's blockbuster romance novel, Kelli O'Hara plays Francesca, an Iowa farm wife who has an affair with Robert, a National Geographicphotographer played by Steven Pasquale.

As with Levy and Swenson, this is also O'Hara and Pasquale's third collaboration. They were in-laws in an early production of The Light in the Piazza.Last year, they were a dysfunctional husband and wife in the musical adaptation of the film Far From Heaven.They're relying on that shared past as they get deeper into Bridges,which opened on Broadway in February.

"The collaboration continues nightly," says Pasquale, sitting with O'Hara in her dressing room before a recent performance. "We've been doing this for weeks, but Kelli and I will still talk about little moments, nitty-gritty stuff. I don't think it's common where you can have that comfort level with an actor."

"I will tell you how debilitating it is to work with someone who gets very comfortable in one type of thing," O'Hara adds. "It's easy to do that in musical theatre, where things are often timed and things don't breathe as freely as they may in a play. If you work with someone like that, it can be frustrating not to feel the freedom to grow and change and shape.

"And when you're with someone you've already worked with, and you know they do let things change, then it's like the door is already open."

Bridgesdirector Bartlett Sher has worked with O'Hara and Pasquale multiple times, and he echoes that idea.

"You're going to go through hard patches, but when you know the person, you know how to deepen because of the conflict," he says, joining the dressing-room conversation. "If I'd never worked with a person before and we ran into a tough patch, I'd be insecure. But here, I'm not dealing with that anymore."

Case in point: Shortly after she opens her heart to Robert, Francesca goes out in a simple brown dress.

"That stupid dress was a big thing," O'Hara says. "I'm saying, 'I want to feel brighter and sexier!' And Bart says, 'Would Francesca feel that way?' It was a head-butt, but I heard him."

Sher jumps in: "She heard me, meaning [she said], 'Fine! If you insist, I'll wear the damn dress!'"

"Yeah, that's how it went," O'Hara laughs.

"Kelli will challenge something very ferociously, and if she challenges it, then I challenge it," Sher says. "But that can't happen if it's too polite. The situation has to be like a good relationship, where if you really know someone, you're able to genuinely explore and not be afraid to go after something."

Mark Blankenship edits TDF Stags (), a magazine about theater and dance. He tweets @iamblankenship.

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