"I practised drinking and gambling for around 25 years to prepare for this role," the Mirror quoted him as saying while telling how he felt about playing suave conman Danny Ocean for the third time.
Clooney also joked about his co-star Brad Pitt at the occasion.
"I'm Brad Pitt - I just look a lot older," he said.
The actor has become slimmer and fitter than ever before. His lean new look is in sharp contrast to how he appeared while filming Syriana, for which he had piled on 30lbs and grew an unflattering beard.
Meanwhile, Clooney has vowed to keep in shape now that he has shed more pounds than he had put on for playing the Oscar-winning role.
"I made a vow to keep in shape. It was frighteningly easy to put the weight on. I did it in a month and that's scary because it takes a lot longer to get it off again. Especially when your body isn't up to working out," the Daily Mail quoted him as saying after shooting Syriana.
The 46-year-old admitted that being fat made him feel old and depressed.
"There was not a moment that was fun about shooting this film. Everybody has a year where they age a decade and this was that one for me," he said.
"It was good for the film and I am glad I did it, but I would never do it again," he added.