In some respects this episode seemed more consistent with the premise of the show as established in Season One, with Andy struggling to catch a break. Once he got his own TV show, he was no longer an "extra", so his problems became different, and -- at times -- less funny.
Ian McKellen's homily on the secrets of acting is hilarious: "I pretend to be the character," he tells Andy in a tone that suggests he's revealing something profound. Then he explains that Andy will be playing a character -- pretending to be John. "You aren't really John," McKellen solemnly informs Andy. The entire scene is brilliant.
For low comedy, there's Darren botching his date with Maggie by sharing way too much information about the plumbing problems he has provoked. Not exactly sophisticated humor... but very funny.
This episode has many moments of laugh-out-loud comedy -- it's Ricky Gervais and Stephen Merchant at their best.