The School of Rock star tackles dark drama in the new Richard Linklater film - about a Texas funeral director who murdered a wealthy widow - and he met the man he plays, Bernie Tiede, behind bars at an East Texas penitentiary.
Director Linklater feared Black would not consider a prison trip, but he was more than happy to sit opposite the man he was about to portray.
Black tells Nylon magazine, ""I was a little nervous about it (meeting). You don't really wanna go to a maximum-security prison.
""(There were) tons of security checks, and the guards are just as severe as some of the inmates. Everyone's sort of on edge... There's a palpable danger in the air. But I had a lot of fans in there.""
The star reveals inmates helped him relax by calling him Nacho, after his character in 2006 comedy Nacho Libre.
But that still didn't prepare him for his meeting with Tiede: ""The pressure of playing a real person kind of came crashing down on me when I met him. I was like, 'Oh, Jesus, what if I don't do it right?'""
The experience made him a fan of the man he's playing in Bernie and now he's hoping the film will speed up Tiede's release - after learning all about the abuse the once-beloved funeral director allegedly experienced at the hands of his eventual victim, Marjorie Nugent.
Black adds, ""It's a heinous crime and the guy definitely deserved to do time, but not all murderers are created equal, and he (Tiede) should be given some credit for all the good things he's done.
""There's precedent for films in the past that have helped people get out, but you worry at the time you're making the film: 'Wait, if we don't present the film properly, will it actually hurt his chances of getting out?'""
Tiede, who was a valued member of the Carthage, Texas community when he shot and killed Nugent in 1996, was sentenced to life imprisonment for her murder.