"Robert had a team of make-up artists working on his stomach to make it appear as if he had a six-pack," Contactmusic quoted a source as telling told the National Enquirer magazine.
"They used all their tricks - including liquid body paint, shading and airbrushing - to make his torso look toned. Whenever they stopped filming, Robert would duck back inside to get touched up by the make-up team. The lighting team helped as well, creating the perfect way to accentuate the shading and highlights," the source added.
The 23-year-old British star is said to be elated with the crew's efforts because he hates working out. The source added: "Rob is definitely not that in shape. He hates the gym and will never be a muscle man."