Fox shed nearly a fifth of his bodyweight to play the emaciated villain in the heavy-duty thriller ‘Alex Cross.’ “When I took on the role in this film I said I wanted to radically change the way that I look, that’s the only way that I would buy me as that character.” It’s a bodily transformation equal to the jailhouse superman that Robert De Niro achieved in Cape Fear and a metamorphosis not dissimilar to Christian Bale’s skeletal appearance in The Machinist.

But the former Lost star wanted to change his body in a more unusual way: he wanted to do it healthily. Whereas Bale simply jacked up his cardio routine and put himself on starvation rations, Fox was determined to do it in such a way that he would have sufficient attention to work hard, think straight and generally be a functioning employee, husband and father.

“We wanted to build a physique that looked natural to the character – not just a gym body,” says Simon Waterson, the trainer who prepares Daniel Craig for his Bond roles and is recognised as the smartest cinematic body-transformer in the business. “Matthew knew exactly what he wanted to look like for the role: menacing and a little psychotic.

“We couldn’t put him on an extreme, low-calorie diet,” says Waterson. “It would have starved his brain of sugar, and his thought processes would have slowed right down. I couldn’t sap him of the energy he needed to work, so everything had to be very balanced.”

In other words, the rapid fat-zapping he undertook may have had crazy results, but it didn’t rely on lunatic methods. Rather, he trained extremely hard but very smart, stuck to a controlled but nutritionally balanced eating plan, and watched his body steadily chisel itself down to nothing but lean, taut muscle.