According to director Fabrice du Welz there are actually only two characters in the film - Marc and Bartel. Welz says that the rest of the people in the film are all variations of the character of Bartel. Notice how everyone in the film is desperate to be close to Marc, just like crazy Bartel.
Welz was reluctant to cast Jackie Berroyer as Paul Bartel, because Berroyer had previously appeared in Welz's short film Quand on est amoureux c'est merveilleux (1999) and Welz didn't think Berroyer was right for the role of Bartel. Welz changed his mind after Berroyer's impressive audition and so he was cast.