ON COMEDY RELIEF: The Requirenment of Seriousness

The actor's primary obligation as the character in the scene is NOT to be funny, but to be very, very serious; to thereby provide the tension in the scene (vis-a-vis the conflict/arument with the other character(s) in the scene) so that the joke in the scene can relieve that tension.

That, in fact, is the origin of the term Comedy Relief.