The actor has apparently been asked to shed 20 kg to play Manya Surve in the film, based on the 1982 incident in which the infamous gangster was shot dead in Mumbai.
John is on a strict diet of fish, sushi, protein shakes, juices and soups to achieve the look, which will be seen in the first half of the film. But he will appear in a more muscular build towards the climax.
"Manya Surve has to look really lean in the first 20 minutes of the film, till he gets into jail and starts working out. So there is an actual physical transformation that my character goes through in the film," said John.
The 39-year-old is taking training from Vinod Channa.
"At times I feel like breaking down. People always talk about losing weight, but it is a very difficult process. If not supervised properly, it can go really wrong. But thankfully my trainer Vinod has been religiously working with me on it," he added.
Directed by Sanjay Gupta, Balaji Motion Pictures’ "Shootout at Wadala", also features Anil Kapoor and Kangna Ranaut. It goes on floors in March.