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".
Mark David 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 (1985). 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.
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 Mark David Chapman did so before killing John Lennon.
Jared Leto filmed his scenes as Mark David Chapman while on a short break from his alternative-rock band, Thirty Seconds to Mars. He lost a majority of the weight he gained while on tour with the band in the year following production. Leto himself has said he's never quite been the same physically since gaining the 67 lbs. for the film.
While most critical reviews were less than positive, critics agreed that the performance by Jared Leto was phenomenal. Leto was praised for his portrayal of Mark David Chapman and how he channeled the inner workings of his psyche in the days leading up to John Lennon's assassination.