I worked at a prison for about a year. My approach ended up like:
I am NP, you are patient. I tried my best to interact on that level, called inmates "Mr." (much to the amusement and derision of correctional officers). If nothing else, reminded me of my role (to provide health care not judgement).
Hope for the best, expect the worst. Told by CO in orientation all inmates are cons, will always be cons, so always try to con you. I hoped a patient wasn't conning me but tried to recognize that he might be - got conned a few times, picked up appendicitis, lymphoma, HIV other times.
Probably best not to know crime committed - when I knew someone was a rapist, child abuser, I had trouble going near them. Though there were some (the self-effacing overdoing maam types) that gave themselves away.
Must think of your safety first. Only one inmate left me feeling threatened (while indirect the verbal threat was quite obvious). My NA was with me, after he left I asked her if she thought I had just been threatened, she said, "for damn sure." So, called correctional supervisor, reported what was said. I did not agree with how they respond to that but it kept me from being injured (he was put back in confinement).
Great advantage over "real world" - when patient (inmate) starts trying to get you to address every problem he has ever had when in for a BP check - happens in the real world all the time - you can say "get out of here" when you've had enough.
In sum, they were a varied, challenging population. Some were people I would not want to ever touch, most weren't. Main reason I left was environment outside of me, the nursing staff, and the inmates.
Hope that helps.