Seth Rogen originally wrote the part of Saul Silver for himself to play. It wasn't until the table read that he realized James Franco would be funnier in the role of Saul.
James Franco's line "It smells like God's vagina" was actually originally improvised by Seth Rogen. Franco told him it wasn't funny, then used the line in the next take.
The diner scene near the end of the film was not in the script. It was improvised on the spot by the actors.
The fight scene between Saul (James Franco) and Carol (Rosie Perez) was, for the most part, improvised. Because of this, Franco was worried that he would hurt Perez, and would ask her for permission to do certain things during the fight. In the end, Perez actually did get hurt, and got a bruise after Franco accidentally bit her too hard on the thigh, but Perez didn't tell him until after filming was done, so he wouldn't feel bad.