Even if the word doesn’t register, you might recognize a Dernsie when you see one on screen. As described in this lively account of Bruce Dern’s life and acting career, it’s his “added dialogue or added behavior that is not in the written dialogue.” It’s a glare or a smirk or a line like the one Quentin Tarantino, one of this documentary’s more frequent talking heads, points to. In Tarantino’s Once Upon a Time in Hollywood, when Brad Pitt wakes Dern, as a napping ranch owner, and tells the barely alert man that his name is Cliff Booth, Dern’s befuddled, funny response, “John Wilkes Booth?” was an improvised Dernsie.

The director, Mike Mendez, best known for horror films including Big Ass Spider!, based the film on recent interviews with Dern done over several years. There are also interviews with his daughter, Laura Dern, and colleagues including Alexander Payne and Walton Goggins. But mostly the camera is close on his face as we listen to an old man with wiry grey hair and a distinctive gravelly voice — Dern is now 89 — tell stories. Fortunately, he is a terrific storyteller, as blunt as you might expect but wittier and warmer. In the way of such documentaries, the film’s tone is adoring, but Dern’s no-nonsense attitude cuts through most of the treacle.

Given his gritty onscreen image, the most surprising aspect of Dern’s real life is his background, illustrated with family photos. During his privileged childhood in Chicago the family had a maid and chauffeur. His grandfather was governor of Utah and a secretary of war under FDR. Adlai Stevenson was his father’s law partner before he ran for president, and young Bruce called him “Uncle.”

As Dern tells it, he was having none of his family’s poshness, and they turned away when he decided to become an actor. He went to New York and The Actors Studio, where Elia Kazan made him do scenes without speaking for a year. Kazan, whom Dern still calls “Mr. Kazan” here, told him his gift was for “behavior.”

Although the film doesn’t explicitly make the connection, that comment goes a long way toward explaining the Dernsies. His most enduring performances, shown in inevitable but judiciously chosen clips, are full of behavior that goes beyond dialogue and gives us masterclasses in acting that range from the glowering, damaged veteran in Coming Home (1978) to the determined older man on a road trip in Nebraska (2013).

His short, early stage career was over when he left for Hollywood. Dern says, “The reason I never went back to the theater is because what we’re doing here” — and he points fiercely at the camera recording him — “is forever.” His vehement tone is bracing throughout, and his deep affection for film is evident.

His comments about his personal life, at least the ones Mendez includes, are brief but direct. He tells a colorful but vague story about his first, youthful marriage which sounds more like a throuple with two women he says were call girls for other women and who fleeced him out of all his money. His famous marriage was to the actress Diane Ladd, a sadder story. Their 18-month old daughter drowned in their swimming pool while he and Ladd were both out and the child was being cared for by a maid. He and Ladd stayed together longer than they should have, he says, “because we shared a tragedy,” but those extra years gave them Laura. Early in Laura’s acting career, Bruce says, he advised her not to let herself be typecast even if it meant passing up work. It’s obviously advice that he, with a range of forgettable villains roles, didn’t always take himself.

The film is fluidly edited and swiftly paced. But Mendez is smart to let Dern do most of the talking, because the interview subjects don’t add much beyond predictable praise, and the style often strains to be more than pedestrian. Mendez’ one formal flourish is to add animation, including puppets, behind Dern’s voice here and there. There’s a Jack Nicholson puppet leading into the clip of The King of Marvin Gardens (1977) and one of Hitchcock, who directed Dern in Family Plot (1976). These work well enough as grace notes and a way to prevent the film from being visually static.

Less successfully, a live-action recreation with Dern playing himself on a hospital gurney after a heart attack is an anomaly and just creepy. And the film makes a great deal of the fact that Dern, a life-long daily runner, has done hundreds of marathons. The frequent references set it up as a clunky metaphor for his professional endurance.   

You could quibble with this or that detail. Dern calls his Cannes Best Actor award for Nebraska “the Palme d’Or for Actor,” which is not a thing. But why nitpick such an amiable film, which feels very much like spending time listening to stories told by a familiar, treasured friend?