Jack Black has revealed it was an "amazing experience" to go to prison to meet murderer Bernie Tiede, who he plays in new dark comedy Bernie and describes as a "gentle giant".
The film is based on the true story of Tiede, a funeral director from a small town in Texas who formed an unlikely friendship with wealthy widow Marjorie Nugent, played by Shirley MacLaine. Nugent was so mean to Tiede that one day he snapped and killed her and hid her body in the freezer.
She was so barely missed that he managed to avoid suspicion for nine months, making excuses for her absence and spending her money to help the local community. He was so loved that even when he was finally charged and confessed to murder, the townspeople rallied round and begged the local attorney not to prosecute him.
Jack revealed: "It was a very strange experience to go to a maximum security prison and meet this sweet guy, who really doesn't seem to fit in there at all, doesn't seem like a hardened criminal at all.
"And I could see why people loved him. He was a real engaging and gentle, charismatic guy. I can see why he had so many friends."
The School Of Rock star felt he couldn't possibly portray a killer masquerading as a nice guy. He had to do justice to the "gentle giant" he'd met.
"I wanted him to be as likeable as he was, in the town. I did my best," he explained. "But I think what was important to portray, was that he didn't really have a release valve. When things made him angry, he never really expressed that.
"Resentment slowly built inside of him over time, for this abusive relationship that he was trapped in, and then he finally just snapped and murdered her.
"It's kind of a cautionary tale - to release your poison!"
:: Bernie is released in cinemas on Friday, April 26.