To add the weight needed to play Chapman, Jared Leto told Jimmy Kimmel he put pints of chocolate Haagen Dazs ice cream in the microwave so he could drink them - with olive oil and soy sauce added "to get me bloated even more".
In the film, Chapman buys a copy of J.D. Salinger's 'The Catcher in the Rye', signs it as 'Holden Caulfield' and writes in it 'This is my statement'. The real Chapman did so before killing John Lennon.
Mark Chapman's meeting with little Sean Lennon and his nanny the day Chapman shot John Lennon was a real incident, though unlike the film's portrayal of it, the meeting took place in the Dakota apartment building.
The name of the actor who plays John Lennon, Mark Lindsay Chapman, is similar to that of Lennon's killer, Mark David Chapman. In 1985, Mark Lindsay Chapman was supposed to play John Lennon in the TV movie John and Yoko: A Love Story. But Lennon's widow, Yoko Ono, who was involved in the production, asked that Chapman be replaced in the role. She considered it "bad karma," because Chapman's name was so similar to her husband's killer's name. Actor Mark McGann played Lennon instead in the TV movie.
Jared Leto who plays John Lennon's Killer, Mark David Chapman, would go on to play the role of Nemo Nobody in the film Mr. Nobody 'Nemo' was the nickname given to Mark David Chapman while attending summer camp at age 16.