Steve Carell's return "was just one of those things that we all vowed to protect," the actor tells PEOPLE

Yes, the cast of The Office heard the return rumors too – and no, they weren t going to spoil the surprise.

“It was so thrilling. We all just flat-out lied,” John Krasinski tells PEOPLE with a laugh at Massachusetts General Hospital Cancer Center’s The One Hundred Gala, held Wednesday in Boston.

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“I lied to Letterman! I have to apologize to him for that at some point. It was just one of those things that we all vowed and had to protect,” the actor says of keeping Steve Carell’s last hurrah under wraps. “Look what happened – it was the best. The ‘that’s what she said’ was the perfect use of Steve.”

Although Krasinski, 33, realizes Pam and Jim fans struggled with the couple’s troubled marriage this season, he says there was a reason behind it.

“[Executive producer] Greg Daniels, Jenna Fischer and I came up with the Jim/Pam storyline this year. Everyone [contributed to] their own. We said, ‘This is what I think has been lacking with my character – I’d love to try this’ and we incorporated almost all of it,” he explains. “We felt that we owed it to our fans who had been watching for so long to resolve these characters in a way that they wanted to see. The reaction to the finale has been really moving, to be honest.”

Three weeks after the final episode aired, Krasinski – who will appear in Monsters University later this month – admits that after nine years spent reporting to set, he’s still getting used to the lack of a day job.

“It’s really weird – it’s a whole lifestyle change. The Office has given me everything,” he says. “I was a waiter when I got the show; I was 23 years old. A decade of my life! Not only has it given me every opportunity in the business, but I wouldn’t have met [my wife] Emily. In a very big, existential way it’s the most important thing in my life.”

Back In Boston

Krasinski and the actress, who will next star opposite Tom Cruise in All You Need Is Kill, were in town to honor 100 individuals and groups who have helped advance the fight against cancer. It was the Newton native’s first trip home since the marathon tragedy in April.

“This is my first time back since the bombings. The way the city bonded together, the way we supported each other and rose to the occasion to send out the message that we are much, much stronger than any one event is what’s always most impressive about being from Boston,” he tells PEOPLE.