Paul Walker was initially considered for the role of Callen in NCIS: Los Angeles before Chris O'Donnell was cast. Walker, known for his role in the Fast & Furious movies, was under consideration when the show was being developed. It is not clear whether Walker declined the part or if the producers chose to go in a different direction. O'Donnell's portrayal of Callen ultimately became very successful, and he played the character for many seasons. Walker tragically passed away in 2013.