There's something engaging about each of these three basic layers of 'Mr. Saturday Night,' although Crystal's comedy far more so than the story or the music. But they exist uneasily together, and wind up undermining one another, not least because the running time of about 170 minutes (including intermission) - way longer than the movie - is too long for a light, sentimental comedy that gets its juice from quick-hit Borscht Belt humor.