Mark Wahlberg was told by the filmmakers that they wanted him to portray the 'older' guy. Initially Troy actor, Garrett Hedlund was considered for the younger role but later Justin Bieber was signed.
“I get this call from Paramount (where they tell me about the story about) an old guy and a young guy. I'm like, 'Cool, let's get Jack (Nicholson). Let's get Robert De Niro. Let's get Robert Duvall.' And they go, 'What about Garrett Hedlund?' I'm like, 'For what?' They're like, 'For the young guy," MTV news quoted Wahlberg as saying.
Actor Mark Wahlberg, who thinks that he is still young, was shocked when he was asked to portray the character of an older person. The Fighter star assumed he was playing the younger role when he first got the offer. The 40-year-old actor even went on looking for co-stars to play the role of the 'older' guy in the upcoming Paramount Pictures movie.