As film festival titles go, “Monogamy” doesn’t exactly grab you. A movie about two people in a committed relationship who don’t sleep with anyone else? Ooh, I’m on the edge of my seat. But when you hear “Monogamy” is the narrative feature debut from Dana Adam Shapiro, who co-directed the extraordinary documentary “Murderball,” you stop being sarcastic for a minute and pay attention. What kind of perspective will a journalist and documentarian bring to a fictional story about human relationships? And will there be a lot of sex?

“Monogamy” stars Chris Messina — last seen as Amy Adams’ husband in “Julie & Julia” — as Theo, a Brooklyn wedding photographer who runs a side business as a professional stalker. People hire him to take pictures of them, without their knowledge, when they’re out in public, as a way of finding out what they look like when they don’t think anyone is watching. (Shapiro says he got the idea from a newspaper story about a business that actually provides this service. This is what happens when people have too much money on their hands.) Theo has some voyeuristic tendencies, then, but that’s probably true of most photographers.

Theo is engaged to Nat (Rashida Jones), an aspiring singer-songwriter who finds Theo’s side business weird (which it is). They’re a good match, coming across as a believable, grown-up couple — Messina and Jones both have a “regular person” vibe about them — rather than a contrived movie couple. Theo seems to be horny all the time while Nat doesn’t seem to want sex very often, but that sitcom cliche feels authentic here.

Theo’s latest client is an anonymous woman (Meital Dohan) who does naughty things in public places. His customers know they’re going to be photographed at some point, of course, but they don’t know exactly when or where. That’s the whole idea. This lady, who calls herself Subgirl, gets off on knowing that there’s a chance she’s being watched RIGHT NOW. Theo is intrigued by her, and by the man he sees her with. He starts to play amateur detective, carefully examining his surveillance photos for clues. The conclusions he draws about Subgirl and her man friend lead him to be jealous and suspicious with Nat, too.

As the film dips its toe into the waters of erotic thrillers (with a nod to “Blow-Up” in particular), we’re left to consider Theo. Specifically: What is Theo’s deal? How does his obsession with Subgirl reflect on his insecurities about marriage and fidelity? Is his chosen career path turning him into a paranoid voyeur, or is it merely providing an outlet for the tendencies he already had? More to the point, what kind of dope messes up a relationship with someone as wonderful as Rashida Jones?

Though the screenplay (which Shapiro co-wrote with newcomer Evan M. Wiener) tends toward the introspective, that’s OK because Chris Messina turns out to be enough of an actor to make it work. His performance is impressively nuanced and sensitive. He isn’t afraid to let Theo be a jerk when necessary, and he conveys this jerkiness in a way that’s relatable and human. As a director and writer, Shapiro demonstrates a knack for authenticity — not surprising, given his background — that makes “Monogamy” a thoughtful and engrossing character study.

B (1 hr., 36 min.; Not Rated, probably R for some harsh profanity, some strong sexuality.)