He sat down with Action News to talk about how his young daughters were the inspiration for the movie.
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The story asks, "What if everything you believed as a kid was real?"
"IF" stands for imaginary friend, Krasinski explained.
"I wanted to do a movie for my kids, forever," Krasinki said.
He and his wife, actress Emily Blunt, have two young daughters.
"The idea came from them," he said. "I would stand in doorways for hours watching these girls just go into a totally different space. They were going to this magical world that we weren't invited to."
"They were not only so joyful, but they were so authentic. They were so brave and fearless in what they were doing," he added.
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Then the pandemic hit. He said he saw their lights start to dwindle a little bit.
"They started to let the real world in and they started imagining a little bit less. I said, 'Stop. This is it. I have to write the movie right now to show them that the magical world that they created, they can always go back to it,'" Krasinski said.
Cailey Fleming plays "Bea," a young girl going through a tough time. She starts to see every person's imaginary friend.
The IFs need help. As their kids grow up, they are looking for new buddies.
Cailey says this film reminds us that, "You don't have to grow up if you don't want to!"
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Bea teams up with Cal, played by Ryan Reynolds, to save the IFs.
The credits on this film feature some of Hollywood's biggest stars.
"I've never had quicker 'yeses' in my career as a director," Krasinski says. "I genuinely got my dream cast. I'll be working off this debt for a long, long time."
Some of those big names include Steve Carell, who is one of the main character IFs called "Blue." Matt Damon, George Clooney, Krasinski's wife Emily Blunt, Reynolds' wife Blake Lively, comedian Sebastian Maniscalco, Maya Rudolph, Awkwafina, and Jenkintown's Bradley Cooper are also a part of the movie.
"IF" is in theaters now.