Dawson Casting: Played straight with Jonah Hill, avoided with Emma Stone, Michael Cera and Christopher Mintz-Plasse. Commentary for the film tells the story that Judd Apatow had to be convinced that Jonah could pull off the role age-wise. Originally the role was for Seth Rogen (who wrote it) but was deemed too old at 25; Jonah Hill ended up in the role despite being just a year younger than Rogen. Rogen admits freely that he's looked thirty since he was about fifteen. Because he was a minor, Christopher Mintz-Plasse's mother was required by law to be on the set when Fogell loses his virginity.
The cops are played by Bill Hader (who was an SNL cast member at the time of the movie's release and popularity) and Knocked Up's Seth Rogen for starters. What's funny is that Bill Hader is also the voice of Mickey the bank robber on the FOX animated sitcom Bob's Burgers, so Hader has played both a cop and a criminal (even though his cop character on Superbad isn't exactly law-abiding either).