I mentioned that Lancelot is a key figure from a thematic perspective.  Here are some of the ways he is significant:

1. In his debate with the devil, he parodies the stage psychomachia (soul struggle).
2. When he tries “confusions” with his father, he parodies Jacob’s deception of Isaac to obtain Esau’s blessing.
3. Trying confusions calls to mind the gold and sliver caskets whose “outward show” is deceptive.
4. The dish of doves, which Old Gobbo gives Lancelot as a gift for Bassanio, represents Christian mercy and peace; it also highlights the other risks in the play.  Lancelot will venture his service in much the same way that the suitors risk perpetual bachelorhood to marry Portia.
5. Lancelot’s shift of allegiance from Shylock to Bassanio reflects the shift from the Judaic to the Christian dispensation.  Think about other conversions—Jessica’s, Shylock’s.  Lancelot parodies Jessica’s conversion at 3.5.21ff by putting it in terms of the rising price of hogs.
6. Lancelot creates an unwanted life—gets a black servant pregnant.  Think about what would have happened if Morocco had chosen the right casket.
7. Lancelot’s use of language:  Compare fools who “for a tricksy word / Defy the matter” at 3.5.66-67 to Gratiano’s use of language—“an infinite deal of nothing” at 1.1.114.  Lancelot’s manipulation of language foreshadows Portia’s use of words to turn Shylock’s bond against him.
8. Lancelot participates in Gratiano’s anti-Semitism.