The Winter's Bone star studied Jessica Yu's short film about O'Brien, Breathing Lessons, and decided he'd have to put his body at risk to play the twisted and emaciated lead authentically.
Hawkes spent weeks perfecting the look, contorting his body - and the role has left him with lasting damage.
He tells WENN, "I tortured myself ultimately because when the script says, 'Because of the curvature of my spine I haven't seen my penis in 30 years', or, 'It may be difficult for certain types of intercourse', you can't disregard that.
"After seeing Jessica Yu's short film on Mark's life and after seeing Mark's body I wanted to try to approximate that. I knew I would have to contort my body. For the spine I worked with the props department to develop a soccer ball-size piece of foam, which I affectionately dubbed the torture ball, that I placed midway on the left side of my back and wanted to portray each body part in the way it was twisted.
"I became obsessed with the film because it offered a specificity about Mark. I'm not four foot eight or weigh 60 pounds; I'm thin but I'm not an emaciated body ravaged by polio. I wanted to get as close to what I saw on screen and insisted from the outset there be no body double or prosthetics. I was hoping to be convincing with what I had.
"I've had issues with my neck and back, but there was no ill will or lawsuits pending as a result of the difficult job!"