From his latest blog post:
I’m having a lot of fun. I just finished my first action scene (yes, there WILL be action scenes!), and am cracking some decent jokes. It’s hard to create jokes for Fraggles, since they have no pop culture references whatsoever. That’s probably a good thing… that means I have a better chance of creating a timeless movie (If I see one more cute iconic character ruined by “Shrekking It Up” for easy jokes, I’m going to go postal).