A documentary about a film critic sounds like the height of perversity — could there be anything less cinematic than watching somebody watch movies? — but of course Roger Ebert wasn’t your average representative of the species. For mostly better and occasional worse, he was the most famous film critic in America, the man who with his TV partner Gene Siskel democratized the way people talk about movies, broadened the kind of movies we watch, and popularized a thumbs-up/thumbs-down approach to quality control that lives on via websites like Rotten Tomatoes.

But that isn’t the reason the gifted director Steve James (“Hoop Dreams”) decided to make “Life Itself,” or why his documentary is tremendously moving even if you haven’t been to the movies in years. As Ebert himself would appreciate, this is simply a great story — a cantankerous young newspaperman who became a passionate and tireless cheerleader for an art form, a lonely soul transformed by love late in life, a cancer victim whose sufferings seemed only to purify him.

Named for Ebert’s 2011 memoir, from which it quotes at length, “Life Itself” is a chronological biography that keeps looping back to the critic’s final months before his death, in April 2013. An unending series of surgeries has left him without a lower jawbone and the ability to speak; when the film opens, he’s heading back into the hospital after fracturing his hip. James dispassionately confronts his subject’s ravaged features and waits for us to get over the shock, which we quickly do. Ebert is adamant about that.

“It would be a major lapse to have a documentary that doesn’t contain the full reality,” he writes the director in an e-mail. “I wouldn’t want to be associated. This is not only your film.”

“Life Itself” amasses a crowd of talking heads from all areas of Ebert’s life, from his childhood in Urbana, Ill., to his colleagues at the University of Illinois’ Daily Illini, to his drinking buddies at the Chicago Sun-Times, where Ebert landed a part. (He won the Pulitzer Prize in 1975, the first film critic to do so.)

Ebert’s timing was lucky: It was 1967 and American film was making its great leap forward, with movies like “Bonnie and Clyde” and “2001: A Space Odyssey” leading the way and critics like Pauline Kael and Andrew Sarris setting the stakes. Ebert never belonged to the high-culture in-crowd, and that seemed fine by him: He was a born newspaperman and a natural writer whose tastes could be far outside the mainstream but whose attitude rarely was.

Filmmakers Werner Herzog, Errol Morris, and Martin Scorsese are on hand to testify to his crucial impact on their careers and even their lives.

There are some fine, funny stories here, but “Life Itself” avoids portraying its subject as a saint, though, and once the film gets into the years with Siskel and the stunning success of their TV review show — it had many names over the years, beginning with “Sneak Previews” — his competitive nature emerges.