One comic scene is that with the horse-courser (one who trains horses to be raced) while Faustus and Mephistophilis are at the Emperor's court. The courser wants to buy Faustus' horse for its great form and build. This of course is the magical horse and accordingly, after agreeing to sell it to the courser, Faustus warns him not to ride the horse into water. No sooner than the courser reaches the edge of town but he challenges Faustus' instructions and rides the horse right into water. All that separated the courser from the water was a "bottle of hay": the horse had vanished leaving nothing behind but hay.
Marlowe employed the same strategy Shakespeare used, that of employing a Fool to add humor and levity. Also, as with Shakespeare, the Fool conveys important information and/or illuminates important aspects of relationships between principal characters: Shakespeare's King Lear and his Fool are the prime example of this strategy. Marlowe uses the fool the horse-courser for levity but also to convey important information. Granted, the information this fool facilitates isn't on the grand scale of Lear's fool but is important to exposing Faustus' character development and the increasing intensity of the falling action that leads up to the climax and resolution.
What art thou, Faustus, but a man condemn'd to die?
Thy fatal time doth draw to final end;
Despair doth drive distrust into my thoughts:
Confound these passions with a quiet sleep:
Tush, Christ did call the thief upon the Cross;
Then rest thee, Faustus, quiet in conceit.
[Sleeps in his chair.]
This quote reveals the important significance of the courser fool and the comic scene. (1) Faustus is given opportunity to lament the changes that are taking over his thoughts, changes that run deeply into his personality: "Despair doth drive distrust into my thoughts: / Confound these passions." His time is running out, his years are almost over, and, "condemned to die," despair drives his thoughts and passions into directions that are unfamiliar to him. This revelation of his character development increases our sympathy for Faustus.
(2) Faustus reveals his deep and hope-filled thoughts about Christ. In an allusion to the thieves upon the cross with Jesus at Calvary, Faustus builds an analogy between himself and one thief. This reveals increasing intensity by showing that he is hoping to be called to redemption by Christ: "Christ did call the thief upon the Cross; / Then rest thee, Faustus." This statement has further significance of its own because it bears heavily upon the climax and resolution when Faustus begs to know how to be redeemed: "I do repent; and yet I do despair: / ... / What shall I do to shun the snares of death?"