Ferrell will appear in four episodes of the show later this season as Deangelo Vickers, a home office branch manager like Michael Scott (Steve Carell).
Speaking to the New York Times, Ferrell explained that he did not have much input into the character.
He said: "[The show's producers] Paul [Lieberstein] and Greg [Daniels] came to sit down with me and I just said, 'I love the show's writing so much that I just trust you guys - why don't you just take a stab at whatever you feel is a good fit?' From that first episode that I'm in, they created, in the style of The Office, this eccentric, bizarre character."
Ferrell also revealed that Deangelo and Michael have a complex relationship.
"Me and Michael Scott, we hit it off immediately," he said. "We're birds of the same feather. And then, in a very Michael Scott way, he slowly starts to resent me because everyone's just kissing up to me."
Ferrell continued: "Slowly, you see in my character that maybe I'm not as good as Michael Scott and am pretty much a weirdo. Everyone's looking at each other going, 'Oh no, we had it so good! We didn't even know it. We knew we had a friend in Michael Scott but we didn't realise, in his backwards way, what a good manager he was'."
Lieberstein was recently forced to deny reports that Ferrell had signed up to join the show full-time after the actor joked that the role had become a "regular" gig.
Watch some footage of Will Ferrell on The Office below: