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Actor John Krasinski, a cast member in the motion picture sport comedy "Leatherheads" attends the premiere of the film at Grauman's Chinese Theatre in the Hollywood section of Los Angeles on March 31, 2008. (UPI Photo/Jim Ruymen)

That didn't keep Krasinski from worrying that he might fall prey to one of Clooney's practical jokes, however.

"Luckily, George was very busy being a director and an actor and it was sort of one of the bigger productions he has ever done," Krasinski told reporters in Los Angeles recently.

"But I will be dead honest and this sounds incredibly crazy, but it's true," he recalled. "I called my manager one day and I thought: 'Oh my God, this is it. This is George Clooney's biggest prank ever. Getting, like, some kid from TV to believe that he actually is on a set with big movie stars and a big crew and he's friends with Renee (Zellweger,) so he got her in on it.'

"After therapy, I realized that wasn't true," he laughed. "But, no, I really did. I was terrified this whole thing might be some weird prank. That's how scared of him I was and, luckily, he was too busy to pull much of anything. ... Everybody was really nice."