Tom McCarthy, the director of acclaimed dramas “The Visitor” and “The Station Agent,” was on set in the locker room of a Long Island, N.Y., high school, working with a colorful trio of actors (Paul Giamatti, Jeffrey Tambor and Bobby Cannavale) who play wrestling coaches in his new film, “Win Win,” an unusual amalgam of buddy comedy, family drama and high school sports movie.

For hours on end, McCarthy had the actors repeat just a few words, doing take after take with only slightly different inflections each time. Ultimately, that day of work became little more than a blink-and-you’ll-miss-it scene lasting a few seconds in the finished film, which premiered Friday night at the Sundance Film Festival ahead of its Los Angeles opening in March.

“Tom is adamant about exploring every possible variation,” said Michael London, who produced both “The Visitor” and “Win Win,” alluding to McCarthy’s perfectionist tweaking on scripts, on set and in the editing room. “It’s painful at times, but in the end it’s exhilarating because he gets to a place most directors never get to.”

That place is visited frequently in “Win Win,” with McCarthy again exploring the poetry of the everyday, in which the lives of normal people take unexpected turns, usually after a chance encounter with a stranger. In the McCarthy canon, authenticity and nuance are prized. Characters are rarely all good or all bad, and they almost never say anything they wouldn’t say in real life.

“I would almost rather risk being boring as a writer than feel I’m manipulating an audience,” McCarthy said in an interview. “For 98 percent of our lives, we have boringness or stillness, and you can allow some of that in the movie.”

There is little, however, that’s boring about “Win Win,” which stuffs a surprising number of characters, genres and even laughs into its unassuming frame.

The movie begins with Mike Flaherty (Giamatti), a New Jersey father, lawyer and part-time high school wrestling coach facing financial troubles. After pulling a slightly underhanded move with an older client to make a few extra bucks, he finds he and wife Jackie (Amy Ryan) in a complicated relationship with the man’s disaffected teenage grandson (newcomer Alex Shaffer), who has shown up unexpectedly from Ohio.

The boy is a gifted wrestler, and “Win Win” soon turns into a high school sports film before veering back to family drama — all while sprinkling in a buddy comedy via a relationship between Mike and childhood friend Terry (Cannavale). If “Black Swan” blends ballet with horror, thriller and other intense genres, “Win Win” does the same with less Hitchcockian ones.

The ability to tell believably human stories has helped make McCarthy a Sundance darling. “Station Agent,” about a relationship between a hot dog vendor and a dwarf, took the festival by storm in 2003 with its mix of eccentricity and empathy. In 2008, “The Visitor,” about a repressed professor’s relationship with a Syrian immigrant, gained a rare standing ovation from the festival’s jaded indie crowd.

It’s fitting that McCarthy keeps coming back. In a festival overrun with young filmmakers trying to tell intimate stories, McCarthy is the man they all want to be. “He makes it look so effortless,” London said, “because he puts in so much effort.”

McCarthy came to prominence as a character actor in movies as varied as “Michael Clayton” and “2012.” While some actors successfully transition to the other side of the camera (Clint Eastwood, Ben Affleck), McCarthy is the rare performer who also pens his own material, baking character shadings that might be left to an actor right into the script. “As an actor, Tom sees things about performance that very few other directors would see, which helps him not only shoot the movie but write it,” said Giamatti on the set.

Even though it’s far more comedic than either of his two previous films, “Win Win” is also McCarthy’s most personal movie. A native New Jerseyian, the 44-year-old wrote the script with his childhood best friend, Joe Tiboni, a first-time writer (and now lawyer and suburban dad) who helped frame the story. And he chose wrestlers because he and Tiboni used to wrestle (“not very well,” Tiboni says) and came across a number of what McCarthy called “damaged kids who had something a little off about them.”

In casting the lead wrestler, McCarthy chose Shaffer, a real-life high school wrestling champion who had never acted before. That concerned pretty much everyone else working on the film, but the director was unbowed. “He had this authentic quality,” McCarthy said, “something that felt honest and original, that didn’t feel manipulated.”

Choosing Shaffer was part of McCarthy’s attempt to walk a fine line when depicting the suburbanites of “Win-Win.”

“I didn’t want to condescend to these people, and I also didn’t want to sentimentalize them,” he said. “I just wanted you to feel what it was like to live with them.”