Originally Posted by WilyMoROCKS
There are some times when Carell is riffing about something (apparently he does a lot of improvising as I've heard in some commentaries on the DVDs) and you can see some of the cast members trying to keep themselves from cracking up, Krasinski in particular.
In the one episode where Phyllis gets flashed, and Carell tells Toby to show him his penis, Krasinski has his hand over his mouth but you can still see a grin behind his hand.
That looks like a show where everyone genuinely has a great time on the set.
I remember reading somewhere that part of the cast has an improv comedy background and the actors are encouraged to stray from the script.