At the time the Office was on air, that cast was a bunch of random nobodies. AFAIK, when the show started, not one of the cast had ever played a lead role in a big-studio movie or TV show. Steve Carrell was the only one who had even played a recognizable role in anything, and that happened right before the show started.
That's half the reason the cast worked so well... it was like watching people you would meet in real life, not watching a bunch of established actors doing characters. If it had been Will Ferrell and Jim Carrey and Amy Poehler, it would never have worked.