The 32-year-old star plays real-life surviver Ralston, who was forced to amputate his own arm after he became trapped under a boulder during a hike in Utah in 2003, E!Online reported.
But the 'Slumdog Millionaire' director says he thought Franco was high on drugs when they met for the audition.
"We saw a number of guys and he wasn't the obvious choice because he's not a look-a-like and he's an urbanite really. He's a city guy like me. He's not a wilderness guy. I wasn't sure at first. We met him and he was so relaxed - he looked stoned to me," Boyle said.
'Milk' star too admits that he won the role because of the memorisation skills he picked up during his stint on American soap opera 'General Hospital'.
"I did have to read for it. (Boyle) had me fly out to LA He didn't give me any material to prepare and when I got there he gave me a page-long speech. He asked if I could memorise and do it right away, which normally would have been difficult but I had just been working on a soap opera, General Hospital.
"I had to memorise like literally 70 pages a day so that little speech was no problem. I don't think he knew at the time that General Hospital helped me get the role," Franco said.