When John Krasinski was 8 years old, he wanted braces more than anything.

That desire was short-lived. ("Turns out, braces aren't as comfortable or as fun as you thought they'd be," Krasinski quips.) But it did spark the name of his imaginary friend, Sam Brace, who would trek with him to the video store near his boyhood home in the Boston suburbs. There, Krasinski picked out the latest action films and comedies, which he proceeded to act out with Sam.

“We would live through these movies,” recalls "The Office" star, who wrote and directed the new family adventure “IF” (in theaters Friday), an acronym for “imaginary friend.” During a recent interview, “someone said, ‘So, Sam directed this movie.’ And I totally blacked out and went, ‘Oh, my God. Sam totally did.’”

John Krasinski and wife Emily Blunt wanted to make 'IF' for their daughters

Not everyone has such fond memories of their imaginary friends. Steve Carell, who plays a giant purple IF named Blue, only vaguely remembers having one.

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"I’m hoping to reclaim that friendship," Carell deadpans. "Perhaps this movie will send me in that direction.”

Cailey Fleming, who portrays the film’s young protagonist Bea, similarly never had pretend pals growing up. But she says she's "never been so moved by a script before."

“I couldn't get through the whole thing without bawling,” says the 17-year-old actress ("The Walking Dead"). “And that's something John and I have in common: We both cry very easily.”

“IF” begins with a heart-wrenching montage in which a younger Bea (Audrey Hoffman) loses her ailing mom (Catharine Daddario). Years later and now a preteen, Bea is sent to stay with her grandmother (Fiona Shaw) while her dad (Krasinski) undergoes surgery. Lonely and wandering around her grandma’s apartment building, she meets a mysterious man (Ryan Reynolds) and a throng of magical creatures, which turn out to be retired IFs whose kids forgot about them when they grew up.

Krasinski, 44, was inspired to make the movie during the pandemic, while sheltering with his wife, Oscar nominee Emily Blunt, and their two daughters, Hazel (10) and Violet (7).

“All the imaginary games that my girls were playing became fewer and fewer,” Krasinski says. “I genuinely saw their lights starting to go out, and they started asking big questions like, ‘Are we going to be OK?’ I said to Emily, ‘This is the definition of growing up,' when you start to make that choice of, ‘Do I let go of all this childhood stuff to be in your real world?’ ”

In writing this story, he wanted to show that "you don't have to choose. That magical world you created is a time capsule you can always go back to."

'IF' is an unexpected 'companion' piece to 'A Quiet Place'

For a visual-effects-heavy kids' movie, "IF" feels remarkably low stakes, as Bea helps find new homes for the imaginary friends and impels adults to reconnect with their inner child. Thankfully, Krasinski says, there was no pressure for a "big action sequence." Rather, he took cues from the films that moved him most as a child, including “E.T.,” “The Goonies” and “Dead Poets Society."

Growing up, "those filmmakers actually gave me credit that I could hold bigger themes," Krasinski says. "That’s really what I took with me into this movie: Don’t shy away from emotionality.”

Although they couldn’t be more different, “IF” has unlikely parallels to his 2018 sci-fi thriller “A Quiet Place,” about a family fighting for survival after an alien invasion. For "Quiet Place," Krasinski drew from his own anxieties as a dad wanting to protect his kids. But "IF" turns the tables to focus on children's worries and fears.

“I love the idea of them being companion pieces,” Krasinski says. "They're both about love and family." Blunt had just given birth to Violet before the first "Quiet Place" film, so “we were in that new parent zone and that felt very authentic. This feels even more authentic," in that "I got this experience of working with my kids and talking to my kids and having them be a part of this from the beginning. It was the most personal thing I have ever done in my career."

Steve Carell says it was 'a no-brainer' reuniting with his 'The Office' co-star

Throughout the making of “IF,” Krasinski’s daughters weighed in on every aspect of the film: He showed them early sketches he drew of the various IFs, and they offered feedback on which scenes to use in the trailer. Their imaginary friends even became characters in the movie, voiced by Maya Rudolph and Krasinski, respectively.

“The pink alligator is Violet’s imaginary friend, and Hazel’s imaginary friend is the marshmallow that lights on fire,” Krasinski explains. “Hazel is a very empathetic person. We were making s'mores one day and her marshmallow caught on fire, and she was emotionally destroyed.”

The movie features an A-list roster of celebrities playing the IFs, including Bradley Cooper, Phoebe Waller-Bridge, George Clooney, Matt Damon and Blake Lively. To portray Blue, Krasinski knew he needed someone who was "sweet" but also "vulnerable." He immediately turned to his beloved "Office" co-star Carell, 61, who has also been his longtime go-to for parenting advice, as a dad to Annie (22) and John (19) with wife Nancy Walls.

"We love each other and we've always worked well together," Carell says. "John is a fantastic director, and there's a reason everybody jumped at the chance to be in this movie. It was a no-brainer for me."

The actors light up when talking about kids' vivid imaginations. Carell is mildly traumatized by this reporter's childhood belief that lobsters lived in the toilet, just waiting to pull us in.

"Now I will always feel like there's a lobster in my toilet," Carell says with a laugh. Growing up, "my son thought that when I was young, the world was in black and white. He didn't think color appeared in the world until he was on the scene. So that's how old he thinks I am!"