Life ain’t easy for Lee (Casey Affleck, left) and his nephew Patrick (Lucas Hedges) in “Manchester by the Sea.” (Roadside Attractions/Amazon Studios)

Writer-director Kenneth Lonergan didn’t add funny moments into “Manchester by the Sea” to keep the movie, about a man coping with grief and loss, from being too sad. He added them to make it real.

“I don’t like it when things have no humor in them, because it usually means they have no life,” Lonergan says. “And comedies with no feelings in them at all aren’t funny. Even the Marx Brothers — their acting is really good. I just think more and more it’s all the same thing — I don’t think there’s a big difference between comedy and drama.”

Lonergan, who’s directed two other sad-funny films — 2000’s “You Can Count on Me” and 2011’s “Margaret” — just wants his movies to be as close to real life as possible.

“I think there are a lot of ways to make movies that are truthful,” he says. “So far, for me, it’s been naturalism. I’m just really interested in the way little details fill your life.”

“Manchester,” which opens Friday, starts with a life-changing moment. Lee (Casey Affleck), a janitor living in Boston, learns his brother (Kyle Chandler) has died suddenly and named Lee guardian to his teenage son, Patrick (Lucas Hedges). Lee returns to his hometown to deal with the ramifications of his new situation.

While Lonergan does mark the major milestones of grief — planning the funeral, meeting with the lawyer — the bulk of the film is Lee coping with the mundanities of a life he didn’t plan on living, things like driving Patrick to hockey practice or dealing with making dinner.

Lonergan was aware of how easily such a story could turn into a movie trope: the resistant guardian who suddenly transforms into a loving, nurturing parent by the time the credits roll.

“People go through incredible pain and I think [resolutions like that are] disrespectful of that experience,” he says. “[Lee], given what I say he’s like, given all the circumstances that surround him — [that ending] would seem like a sentimental fairy tale.”

Lonergan’s somewhat detached directing style helps keep that kind of sentimentality at bay. He deliberately refrains from nudging the audience to think or feel a certain way.

“There’s what’s happening in the story, and then there’s my opinion about it,” Lonergan says. “My opinion about it might be different from your opinion about it, and if the story bears any resemblance to real life, then our opinions probably will be different. People having a different opinion means I’ve been successful.”