Balthazar was an untrained donkey during most of the filming, which made Robert Bresson's work a real challenge. The only scene for which the donkey was trained was the circus math trick.
In an interview, Robert Bresson said he was inspired to make the story after reading a passage in Fyodor Dostoevsky's novel The Idiot, in which the main character the Prince mentions his special fondness among animals for the lowly donkey.
Title translation: Balthazar, at Random.
Ingmar Bergman admitted to not enjoying this film (though he praised Robert Bresson's follow-up film Mouchette (1967)).