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.
Michaël R. Roskam spent six years working on the project, back then, Matthias Schoenaerts started working out to have a little basis to work from once he knew when they would shoot the film. Before "Bullhead", Roskam and Schoenaerts had worked together on the short film The One Thing to Do (2005).
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.