Longtime friend and collaborator, director Ryoo Seung-wan has spoken of how he greatly appreciates Jung's stubbornness when providing creative input in his films' action scenes. Jung, who has appeared onscreen in minor and supporting parts throughout his career, has had his most significant acting roles in Ryoo's movies, particularly as the villain in action-comedy-fantasy "Arahan" (2004). In 2006, Ryoo cast Jung and himself in the leading roles in "The City of Violence", about two friends who reunite in their hometown and go after the bad guys who killed their friend. Jung said he had wanted to be in a film that was "simply action" (without romance or suspense) and thought, "This was my last chance [to appear in an action movie]. So I wanted to show everything I had." In 2005 Jung (by then already in his forties) made his debut as a professional boxer in the welterweight division. He also opened a fitness center with TV host Lee Hoon, called "Double H Multi Gym." Jung married Indonesian-Australian model and former "MTV Asia" VJ Nadya Hutagalung in 1998, but the union ended within a year; Hutagalung later said the marriage wasn't officially registered.