Stan did time before when he was youngerStan had a very dysfunctional childhood growing up (he mentions half-sarcastically to Bill at one point that his parents argue all the time), and often into trouble as a result, because he was a problem child. It finally got so bad that he ended up committing a very serious offense that landed him in jail at a young age. This was his wakeup call: the experience of being in jail not only scared him straight, but scared him shitless as well; he witnessed all of the things that go on in jail, including those he keeps mentioning to Bill about prison rapes and sex slavery. When he and Bill are arrested and sent to jail, the memories of his previous time in jail are all he can think about, and therefore he's terrified of reliving those experiences all over again.
- He and Bill are just starting college in the fall, meaning that up until very, very recently, Stan was a minor, and absolutely would not have been sent to prison unless he committed some heinous crime like murder. No, Stan's behavior is not that of someone who's experienced it, it's the behavior of someone who's seen too many movies and is jumping hysterically to conclusions. One might as well posit that he spent his youth watching southerners marry their sisters, because he talks about that in the exact same tone.