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."