Last Days is loosely based on the life and suicide of the doomed NIRVANA star but Pitt was nervous of living up to the god-like status of the rock idol both musically and personally.
And he was desperate to avoid fans thinking he was cashing in on the tragedy to further his own music career.
He tells British magazine Hotdog, "I had a lot of fears. I wasn't sold on doing it.
"My biggest fear was my music. I take it seriously and I didn't want to seem as if I was doing it for personal gain.
"My other fear was the fans. Playing someone that you really idolised, you just have this feeling of like, 'No way! Get someone good!'
"I'm not playing him - Kurt - and not using his name of facts or dates, it allows the audience to see a human being in that situation as opposed to Kurt Cobain."