I often find myself being totally knocked for six when comedians stop the funny and have a little honest moment.

Is it because they are funny, funny, funny, funny, serious... So it catches you off guard? Or are they just damned good at it... does being funny naturally lend itself to being particularly good at acting all round?

This was the direct inspiration for the thread:
Mitchell and Webb - Old Holmes
http://www.youtube.com/watch?v=Pp02ubGuTIU

Any other examples?