Lee Mack seems to be one of the old school naturally witty comedians. A lot of good comedians are actually just really good writers who might not necessarily be that funny in real life, whereas I think people like Lee Mack are just really quick witted (as well as being pretty good are writing a routine too).
Because they're massive geeks about it and are constantly tweaking the act to get the maximum impact for each joke. It'd probably be pretty interesting actually, because it's not like being an actor, where you literally have to deliver exactly the same performance every night.
