Matthias Schoenaerts had to cultivate 27 kilograms of muscle mass to apply himself to the role of Jacky Vanmarsenille, he prepped for the role for two years, committing to a diet of 3000 tins of tuna, several hundred chickens and plenty of oats, rice and pasta combined with an exhaustive weight lifting regimen. In addition, he also wore a prosthetic nose.
Matthias Schoenaerts studied the Limburgish dialect to ensure he was performing with authenticity, so he wouldn't offend the natives.
According to Michaël R. Roskam, his inspiration for the film was based on a crime scene in Belgium known as the Belgium Hormone Mafia.
During the New York Comic Con in October 2013, the actor Steven Ogg who plays Trevor Phillips in GTA V, revealed that his inspirations to play the character were Matthias Schoenaerts in Bullhead - La vincente ascesa di Jacky (2011) and Tom Hardy in Bronson (2008). Hardy and Schoenaerts worked together in Chi è senza colpa (2014), also directed by Michaël R. Roskam.
Bruno's vegetative state is never explained because Michael Roskam simply never thought of one. After the fact, Roskam offered several theories including disease, a failed suicide attempt, or being beaten by Jacky.