The Rock star thought it would be a challenge to play Gacy in Dear Mr. Gacy - the movie based on the book The Last Victim by criminology student Jason Moss, who corresponded with the sick killer while he was on death row - but he had no idea just how much the research would play with his mind.
Forsythe tells BlogTalkRadio.com, "I began to have crazy dreams and very realistic. The dreams that I was having were basically (from) Gacy's perspective... It was a lot of dreams, where I would be driving along in a car and looking out the window and I was seeing faces of boys standing on corners talking and just seeing them from his perspective.
"It's chilling because these dreams were exactly what John Wayne Gacy would do. I put together a slide show of the photographs of his victims and I couldn't get through that slide show without emotionally breaking down. It was so disturbing to see the faces of these young innocent boys that he lured in under the guise of being a nice guy."
Forsythe also struggled to get Gacy out of his life after the movie - because so many people wanted to talk to him about the portrayal.
He adds, "I was conscious of how disturbing the journey would be. This was a journey from hell for me and I'll never look back and have anything to do with John Wayne Gacy again because it's over for me. I literally just turned on the Beach Boys and went back to being a blond. I tried to surf my way out of it in a way and go to a whole other place.
"I almost feel bad about it because the doors that I opened up in Chicago research-wise, I had to stop talking to those people for at least this past year because I just didn't want to talk about John Wayne Gacy. These are good people in their own right, particularly his best friend Barry (Boschelli), who was a victim of John... He was the nicest guy and I owe him so much for the understanding of who Gacy was but I found I couldn't return his phone calls after awhile. I'm going to call him again but I had to get away from Gacy."