Well, judging by how some fans of The Office reacted to it, perhaps secretly is the wrong word. John Krasinski played Jim Halpert throughout the entirety of The Office, but his star began rising quite early on and the actor had numerous roles in feature-films. During the third season of the series, Krasinski joined the cast of George Clooney's LEATHERHEADS, but this presented a problem when it became necessary for the actor to trim his trademark Office hair.
In Andy Greene's book "The Office: The Untold Story of the Greatest Sitcom of the 2000s" (h/t Collider), The Office's hairstylist Kim Ferry recounted how creator/showrunner Greg Daniels initially didn't want Krasinski to cut his hair, but Krasinski reached out to Ferry for assistance.
It would have meant cutting his hair into a 1920s hairstyle. But continuity-wise and contractually-wise, actors are obligated to keep their hair how it is for a series unless they get producer approval, of course. But it would have been cutting his hair super short, which would mean getting rid of the wings on the side and his long hair. He came to me and said, ‘Could we do a wig?’ He went to talk to them and said, ‘I really want to do this project.’ And they were like, ‘I think it would be obvious that it was a wig.’ Greg really felt like it won’t look like him.
John Krasinski had Ferry make a wig for him in secret and planned to talk to the producers before revealing the existence of the hairpiece. Ferry explained how she walked into Greg Daniel's office and saw Krasinski speaking with Daniels, all while the wig sat on Daniel's desk. "John told me later that Greg said to him, ‘John, I’ll know if it’s a wig. You can’t fake that kind of thing,'" Ferry recounted. "As he’s staring at him with the wig on. And then John’s like, ‘Really? I don’t think you would,’ and he takes it off right in front of him.’ And then Greg said, ‘You win, I give you full permission to wear the wig.’ When I came in [Greg] said to me, ‘You guys have a lot of balls.’ For a minute I really thought I was going to get fired." Although Greg Daniel's was fooled in by John Krasinski's wig in person, it seems that fans of The Office weren't as they've frequently brought up just how strange his hair looks in the third season.
John Krasinski will next be seen in A QUIET PLACE: PART II, which is currently slated to hit theaters on September 4, 2020.