Director - Bryan Singer

Christopher McQuarrie wrote only one line for this scene -"Give me the keys, you f*cking c*cksucker!" - it was up to the individual actors to deliver it however they wanted. McQuarrie actually plays the cop speaking with the suspects and both his line to Fred Fenster (Benicio Del Toro) "In English please?" and Del Toro's reaction were unscripted.
According to interviews on the DVD, the laughing during Del Toro's delivery was due to his constant farting while filming - boys will be boys.