By Ken Tucker
Updated June 01, 2010 at 06:16 PM EDT

John Cazale possessed a soulfulness and vulnerability that few actors are able to tap into; in the first two Godfather movies, in Dog Day Afternoon, The Deer Hunter, and The Conversation, Cazale glowed in the background. He wasn’t a scene-stealer — he enhanced and heightened the mood of each scene in which he appeared. Tonight, a wonderful, brief documentary, I Knew It Was You: Rediscovering John Cazale, gives Cazale his due.

Cazale died from cancer at age 42 in 1978. As the film says, “he was only in five films, and each one of them was nominated for [an Oscar] best picture.” Cazale was the sort of performer rare these days: Primarily a stage actor, taking small movie roles when the material, the director, and the rest of the cast interested him. He didn’t seek stardom, and died before any sort of sizable, adoring cult could build him around him.

The title of director Richard Shepard’s documentary paraphrases the famous line in The Godfather Part II, in which Cazale played the weak older brother Fredo.

In I Knew It Was You, you’ll see and hear Al Pacino, Robert De Niro, Francis Ford Coppola and others praise Cazale’s subtle gifts, his diffidence, his meticulousness. He had a romantic relationship with Meryl Streep that was cut short by his death; Streep’s comments here are all the more moving for being so simply and briefly stated.

The line that followed Pacino’s famous line to Cazale’s Fredo was, “You broke my heart.” This documentary is likely to break yours, even as it fills you with inspiration to know that such as artist could do such good work in however brief a time he lived.

I Knew It Was You premieres on HBO tonight at 8 p.m. EST, with multiple re-airings.