Jared Leto gained 67 pounds for the role. At times he was confined to a wheelchair due to so much added weight.
Some of the side effects Jared Leto experienced as a result of his rapid weight gain included gout, elevated cholesterol levels, back pain, and an irregular heartbeat.
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.
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."
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.