The "Transformers" star hit rock bottom while making the movie, about a man with the genetic disorder who dreams of becoming a wrestler, when he was arrested following a drunken rant in Savannah, Georgia in July 2017.
Shia was ashamed with himself after the incident, which could have cost Zack his first movie role if production had been shut down, and promised to give up alcohol while finishing the film.
"I can't look anyone in the eye," he tells The Guardian of how he felt after the arrest. "And I can't come to terms with it. And no one's talking to me and I'm talking to no one, and I know my life has exploded again. And I am in deep shame".
Zack tells the newspaper that the pair have stayed in touch since, and he's been keeping an eye on his movie star pal - who is now more than two years sober.
"I have been changing Shia's life," he explains. "That's how I do things. And Shia has proved to himself he doesn't have to act out. So I'm going to keep taking the chance of being close to him".
Pledging to stand by his notorious pal, he adds: "A lot of people have come to me talking about Shia, and they always tell me: 'Stay away from Shia.' But I am not going to do that. My answer is: 'No.' Because people don't understand the real Shia".
"The Peanut Butter Falcon" is in cinemas now.