For its first half-hour “God’s Fiddler,” a documentary about the violinist Jascha Heifetz, one of the 20th century’s greatest musicians, seems as if it’s going to be as cold as his playing was sometimes accused of being. But it warms up a bit when his former aides, accompanists and, especially, students start sharing their memories of this quirky, aloof man.

Heifetz, born in 1901 in Russia and acclaimed there while still a boy, had the good fortune to leave that country for his first United States tour just as the revolution was breaking out, and he lived his adult life in the West. The film is full of musicians’ attestations of just how far above every other violinist he was. Many also rebut the notion that his playing was cold: his onstage demeanor may have been stoic, they say, but the music sizzled.

The film, though generous with doses of Heifetz in performance, isn’t entirely successful at illuminating the man. For instance it mentions only late and in passing his two wives and three children.

Anecdotal tidbits shed some light. He used to summon his assistants to his study with a bicycle horn; living in California, he became so concerned about smog that he converted his car to run on batteries. Most revealing, though, is the effect he had on others.

“When everyone had entered the classroom, we all sat nervously waiting for his first words, which generally would be, ‘Who’s ready?’ ” says one former student, Sherry Kloss. “I almost passed out when I heard those two words. Who’s ready? How could anyone ever be ready to play for Jascha Heifetz?”