In “The Last Five Years,” Jamie and Cathy (James Gardiner and Erin Weaver) slowly realize their marriage won’t last.
In “The Last Five Years,” Jamie and Cathy (James Gardiner and Erin Weaver) slowly realize their marriage won’t last.

Actors don’t have to like their characters, but they do have to understand them. When it came to his role as a self-centered adulterer in “The Last Five Years,” now at Signature Theatre, it took James Gardiner years to succeed.

“It’s very interesting to examine this piece as a married guy,” says Gardiner, who plays the musical’s leading man, Jamie. He last played the role a decade ago, when he was 19.

“At that age, I saw Jamie as the bad guy,” he says. “Now that I am married it’s just changed. It’s not black-and- white and there are issues you have to deal with.”

“Five Years” has a simple plot: Two people fall in love, marry, fall out of love and divorce. What sets it apart from other musicals is the structure: The show is mostly a series of solos, with Cathy (Erin Weaver) telling the story backward and Jamie telling it forward.

In the first song, Cathy is mourning the death of her marriage after her husband’s affair. In the second, Jamie is thrilled by his great first date with Cathy. The two share the stage only once, in the middle of the show, at their wedding.

The structure serves as more than a gimmick (and a way to keep the show from having such a downer ending). Gardiner says the opposing timelines make it clear that the two characters, as much as they love each other, “are passing ships who never really connect.”

Gardiner’s character bears the burden of being the one who cheats. But he’s not the villain, just a screwed-up human in a tough situation. “Some people think the infidelity ended the marriage, but that’s just the icing,” Gardiner says.

Though Gardiner has run into audience members who side with one character or the other, he feels the show’s nuance lifts it above “Team Jamie” and “Team Cathy” pettiness.

“The first few days of rehearsal, I kept saying, ‘God, Jamie’s an asshole,’ but [director Aaron] Posner said, ‘Stop judging him,’ ” Gardiner says. “My biggest fear was looking out in the audience and seeing people who are disgusted. But I don’t feel that anymore.”

Signature Theatre, 4200 Campbell Ave., Arlington; through April 28, $29-$86; 703-820-9771.