ALL HOLLYWOOD CELEBRITIES have a real-life counterpart to their on-screen persona. It’s part of the mythmaking and artifice of movie stars. And we enjoy glimpsing the spoils of their success — the Malibu palaces, the glittering company they keep — as much as we relish discovering that the distance dividing us from them is not so great. If, at one moment, they are captured in high-definition for the screen, then they are also delivered to us by harried paparazzi shots, red-carpet pictures and social media. Mortensen’s life, as much as it is publicly shared, is something of an outlier to the cliché. He is thoroughly uninterested in playing the game of status and vanity. Or, as Mortensen said in an interview with a small Idaho newspaper last year: “What people might generally regard as typical ‘Hollywood’ behavior (seeking maximum attention and hobnobbing with movie people at all times), I am not really drawn to that.” That he didn’t achieve fame until his early 40s helps. His first wife is Exene Cervenka, the lead singer of the punk band X; the two have a son, Henry, who is now 30, and at one point ditched Los Angeles for a life together out in Sandpoint, Idaho. Mortensen told me that he spent the night before we met riding the subway to the Bronx, to the New York Botanical Garden. He ended up talking to a middle-aged Mexican man, a contractor, on his way home from the job. Surely, I asked, the guy knew who you were? Mortensen shakes his head, “No, I don’t think so.”

He’s a regular guy — except he’s not. There’s something about Mortensen that is difficult to describe, because who he is, paradoxically, is almost entirely about what he isn’t. The empty charm and insecure braggadocio often present in his peers are unsettlingly, though wonderfully, absent in him. He is, in such a superficial medium, able to transmit the feeling of a soul. In “Captain Fantastic,” there’s a moment where Mortensen, who plays Ben, the father of the family, must tell his children they won’t be returning home to the woods. The children are incredulous; they believe they can still persuade their father to change his mind. But Ben has decided. Mortensen’s body hardly moves: One eyebrow lifts, pulling the rest of his face up to acknowledge that what he has said is final. Then he remains impassively still — calm but resigned. As he speaks in a tumbled monologue, you can see more than you can hear the accumulation of events that have led Ben here, various emotions briefly flickering across his face. It’s scenes like this, so fleeting and yet so profound, that still make Mortensen a bit of a mystery to me.