Shia LaBeouf planned to join the Peace Corps because he thought his acting career was over

Asked if he thought about casting anyone else as his dad, he admitted: "I didn't think I'd be able to play him, because I was not in a spot where people were like, 'Hey, let's put some money on this kid's back and have him carry a movie.'

"I thought my acting career was done. I was going to join the Peace Corps."

The 'Lawless' star had eyed Mel Gibson for the part instead - and even told his estranged father that the 'Braveheart' actor was going to portray him when he sought permission for the story to be played out.

He continued: "So I sent it to Mel Gibson and luckily he never emailed me back and it gave me an opportunity. I thought he was the guy to play my dad, and my dad was thinking along the same lines.

"And it's one thing to want to play your dad, it's another thing to go stand in front of your father after seven years of not talking and go, 'Hey, man, I'm going to play you', when there's contention already. So I lied to him and told him, "Mel Gibson's going to play you. Sign right here.

"So my dad signed the paper under the auspices that he was going to be played by Braveheart."

The 'Transformers' star - who is played by Noah Jupe and Lucas Hedges in the film - recalled how he'd reached rock bottom when he started writing the film.

Speaking about why he wanted to make the movie to fellow actors including Tom Hanks, Jamie Foxx, Adam Driver and Adam Sandler in a roundtable for The Hollywood Reporter, he said: "My back was against the wall. I was nuclear at this point. It felt like survival, like there was no other way to go. I didn't have a lot of people talking to me.

"I was in a mental institution. And I also had a doctor who was pushing me to explore these dirty parts and write them down."