Hypocrisy is just as alive in the prisons as it is in the churches!
Usually, preaching the gospel in prison means that most of your hearers know they have 'messed up' their lives, and they know that you know they have. So there is often a more honest starting point of admission of sin than sometimes we see in our respectable churches.
However, human nature does not change because it is put behind bars, and hypocrisy is alive and well in prison too. Nowhere do you see this more than in some 'ordinary' prisoners' reactions to vulnerable prisoners - known as 'VPs' - especially those who are VPs because of crimes involving sexual misconduct.