Jack Black's performance as Dewey is definitely one of the most perfectly cast roles, and that makes a whole lot of sense, seeing as screenwriter Mike White wrote the part, and the whole film, for Jack.
Mike, who played Ned in the film, spoke about how he came up with the idea for the movie based on Jack,. He was really funny but the part was not written for him. And I just kept thinking there’s so many different colours to his comedy than people have seen, at least in terms of the movies he’s headlined.
I just had this image in my head of him jamming with a bunch of little kids, and while I was kind of afraid of what the Paramount version of that could be, I couldn’t shake the idea. So I just wrote it." And it's a good thing that Jack was happy to take up the role too, because Mike explained that without him as Dewey, he wouldn't have wanted to continue with the movie, saying:"Had he not wanted to do it, I probably would have thrown the script away. It was so inspired by just knowing him that I wouldn’t want to see anyone else do it."