Nichols isn't being paid for her work with the FBI. During her time she has participated in half a dozen child-sex stings around the world as a volunteer.
In the interview, the actress told how she would write messages into a popular app that men use to seek sex with children, pretending to be the child's father or parent.
She would then feign the voice of a young child on the phone, usually sounding drugged and naive, giggling on cue.
"If good people don't know about it, it will keep happening, because good people are the only ones who will do anything about it," she said.
She recalled one time she played a trafficker who set up a child sex party.
The target "was 38, looked like a real estate agent or something".
She told Marie Claire: "These guys look like normal people. And you're pretending that you just happily and eagerly set up children for them to have sex with."
She went on: "To watch his eyes (at the mention of an underage child) you want to kick him in the balls and beat the hell out of him."
The actress said technology was a big part of catching the paedophiles, but it also constantly changes. One project she worked on five years ago involved detectives posting ads for sex with children on Craigslist.
"Within 15 minutes, we had 30 appointments," she said. Then there was Backpage, "the freaking Disney World for paedophiles".
During her time volunteering for the FBI, Nichols had been busy with her day job โ filming Riverdale, which she left at the end of season four, and also working on Sprial, thereboot of Saw, out next year.
In fact, it was when her career slowed down back in 2012 that she began to work in the FBI's Operation Underground Railroad and local law enforcement division. Before then, she played the role of a cop in both CSI: Crime Scene Investigation and Law & Order: SVU.
Then in 2014 she started a non-profit organisation, Foundation for Slavery Free World, and has since become a regular spokesperson to the public about the subject.
The incredible story about what she has been doing as a Hollywood star turned vigilante will make it to the small screen, with Sony in the early stages of development.
Nichols will executive produce and most likely star in the adaptation.