The Ocean's Eleven and Eastern Promises villain piled on 40 pounds (18.1 kilograms) to play infamous French gangster Jacques Mesrine in the film and then had to lose all the weight to play the thug as a young man.
Wiry Cassel called in experts when his efforts to shed the pounds became a little too easy.
He says, "I was helped by my doctors. For the first time in my life, I had bad cholesterol. I was freaking out, but by the time I finished the movie, the weight was gone and so was the bad cholesterol.
"I actually had a problem keeping the weight on. I had an assistant coming with this huge milkshake every day. When we completed the first (part of the) movie, I actually started to lose weight too fast and I had to stay on a certain kind of diet."
Cassel's efforts have already been rewarded in his native France - he picked up a Cesar award for his portrayal of Mesrine.
The film was a labour of love for the actor - he spent seven years trying to find the perfect director and raise the $50 million (£33.3 million) required to make the gritty four-hour epic.
But the wait cost him the chance to appear onscreen with his real-life father - Jean-Pierre Cassel died before the film started, so the Derailed star turned to family friend Michel Duchaussoy to play his character's dying dad.
Cassel admits the scene in which Mesrine bids his father farewell was packed with emotion.
He tells the Los Angeles Times, "The day we shot the scene it was terrible. There was a tension on the set. We shot that really fast, maybe three takes. Then I shook his hand and said, 'Ciao'."