The actress plays a woman accused of murder in the new Russell Crowe film, and actually spent time at the Allegheny State Prison in Pittsburgh, Pennsylvania in an effort to get into her character's head.
She says, "My character is incarcerated at the Allegheny County Jail where we actually shot the movie, so of course it was really helpful in terms of getting into character. I got to sit with the inmates. I got to be locked in a cell which lasted about two minutes before it was like, 'Get me the hell out of here!'
"They don't have bars on their doors; it's all heavy steel doors with a little window, so you feel not just incarcerated but also institutionalised. You feel like you're gonna get a lobotomy in about five minutes, so it was very easy to imagine the isolation of being there for real - and the depression.
"There is no outdoor space in Allegheny; you never see the sun. I actually believe that there are people that should be separated from society and I believe there are bad people. I sat with about 40 or 50 women on this one cell block... It's really not a fun place to be. Most of the women I met with were either drug offenders or parole violators; not a lot of super violent women on this particular cell block, but some women were there for up to three years."
And Banks rewarded the women by buying them all new prison attire after trying it on for herself.
She tells WENN, "We had all prison issue costumes when we went to the jail and asked them for all their old stuff. We took it and we bought them new stuff. I wore all the clothes, socks, underwear and it's itchy, it's scratchy, it's thin, it's gross.
"The worst part about it is it has an elastic waistband that cuts into you and I'm tiny so you wouldn't think it would bother me but it did. I said, 'How do you guys deal with this?'"