Just who are Michael and Mary, the philandering husband and wife in Azazel Jacobs’ comedy-drama “The Lovers”? Usually, if you have to ask such a question, the movie wasn’t any good. “The Lovers,” however, seems different. We don’t understand these two. We may not even like them. And yet, we’re fascinated by everything they say and do.

This uneven, sometimes frustrating but ultimately engrossing movie wouldn’t work as well as it does without its two stars, Tracy Letts and Debra Winger. Letts, the Pulitzer-winning playwright (“August: Osage County”), is emerging at the age of 51 as a prodigious film actor, with seven movies to his credit since 2015 (including last year’s excellent “Indignation”). Winger, 62, seems to mount a comeback role every decade or so, most recently in 2008’s “Rachel Getting Married,” and she can add this one to her list. They’re marvelous together as Michael and Mary, who are squandering middle age on a miserable marriage and, in the process, spreading their unhappiness to others.

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Michael is a corporate drone who brightens his lunchtimes with Lucy (Melora Walters), an emotionally needy ballet teacher. Mary, meanwhile, dallies with the much-younger Robert (Aidan Gillen), an aspiring writer. These two side dishes might seem to be the lovers of this movie’s title, but just when Michael and Mary get ready to divorce, something odd happens: They interest each other again. Sexual desire rekindles. In short order, they’re planning quick trysts and making steamy phone calls to each other, leaving Lucy and Robert to ask the agonizingly ironic question, “Are you seeing someone?”

“The Lovers” often feels like light farce, as when Michael and Mary accidentally snuggle in their sleep, then awaken in simultaneous horror. At other moments, though, it can be emotionally bruising. Michael, in particular, is a merciless liar, and his girlfriend’s tears are a recurring motif. When the couple’s college-age son, Joel (Tyler Ross), reluctantly comes home for a visit, the film’s kookiness quickly gives way to the bitter tone of an Albee play.

It’s possible that writer-director Jacobs (“Momma’s Man”) isn’t sure himself how to feel about the characters he’s created. We never doubt they’re real, though. Like so many of us, they’re complicated, contradictory and, perhaps, unknowable even to themselves.