Nerves, it makes him very nervous being with proper Actors and he doesn't think he deserves to be acting and a proper actor should do it, the first series was just a favour for a mate !!
I loved his "An Idiot Abroad" series (the episode with him and Warrick Davis on the boat in China remains one of the funniest things I've seen on TV). I tried to get into "The Moaning of Life" but it didn't stick.
Placing America (Route 66) in the second season was perfect. All the exotic locations of season one, and then good old white trash America. Yep, that's us.
I'm finally watching Derek on Netflix. It's hilarious that Gervais and Pilkington exchanged their "characters" for that show.