Val Kilmer is the subject, the star and, against steep odds, the unquenchable life force of “Val,” a feature documentary, playing in theaters, by Ting Poo and Leo Scott.

For anyone who has wondered whatever happened to the magnetic, volatile, seemingly guileless and sometimes otherworldly actor who was launched into Hollywood stardom nearly four decades ago by his performances as Iceman in “Top Gun,” Jim Morrison in “The Doors” and Doc Holliday in “Tombstone,” the answer on screen may come as a shock. What happened—after long-running career turmoil as well as financial and personal distress—was ravaging cancer, which left him with a tracheostomy and has severely limited his ability to speak. So the film takes some easing into. There’s also the minor but still startling matter of visual and informational clutter. It’s as if one of the Collyer brothers invited you in to show you a few things he’d grown attached to over the years, except that Mr. Kilmer’s things go beyond books, posters, newspapers, magazines, journals and roomfuls of cherished objects to include an exhaustive home-video record of his private, public and professional life to date.

Exhaustive but not exhausting. On the contrary, the fuzzy footage provides fascinating behind-the-scenes glimpses of Mr. Kilmer and his co-stars between takes on movie sets. And whatever he isn’t able to say in his own voice, straight to this latest camera confronting him, he says in voice-over passages written by him and relayed with warmth and feeling by his son, Jack Kilmer, who is seen in the process of recording them. (The unspoken connection between father and son is ineffably moving.) The result is a documentary that keeps drawing you in, even when you think it’s keeping you at a certain distance, a one-of-a-kind portrait of a one-of-a-kind artist who, through good times and dreadful ones, has remained devoted to his art.

Jack Kilmer, who recorded voice-overs for the documentary

Jack Kilmer, who recorded voice-overs for the documentary

Photo: A24/Amazon Studios

“I’ve kept everything,” he notes at the outset, “and it’s been sitting in boxes for years.” He amassed his archive, as he frames it, in order to tell a story about acting—“where you end and the acting begins.” That’s elusive territory for any performer to define. For Mr. Kilmer it has been, at various times in his life, a magical zone, a seductive mirage or a Sargasso of obsession, but always a sacred space of which to be worthy—even when he played Batman in the 1995 “Batman Forever” and struggled unavailingly to project a nuanced performance from inside the rubber suit. Now, he says, referring to the larger story of his acting, “part of the profound sadness is that it’s incomplete.”

Val Kilmer

Val Kilmer

Photo: A24/Amazon Studios

Sadness claims its place in the film, as it has in his life, a response to mounting losses of love and possibility. And not just sadness but irreparable grief over the death of a beloved brother, Wesley, who drowned at age 15 during an epileptic seizure. Yet the dominant colors of “Val” as a whole are steadfast aspiration—in his beautiful teens he was the youngest applicant ever accepted as a drama student at Juilliard—and tacit gallantry in straitened circumstances. Sometimes he gets “the blues really, really bad” about keeping himself solvent by flying around the country to sign autographs for his fans: “It’s like the lowest thing you can do.” But it isn’t low, any more than it’s high. It’s making the best of a role he never sought, and playing it as he’s always tried to do, in the urgency of the moment.

Write to Joe Morgenstern at joe.morgenstern@wsj.com