This is my favorite scene, bar none, from Extras. It's Ricky Gervais (as Andy Millman) and Sir Ian McKellern as himself!
Watch him school Ricky Gervais's character on the most subtle tricks of the trade. Watch Gervais (as Andy Millman) struggle to prevent his trademark inscrutability from giving way to slack-jawed incredulity. Watch him fail to conceal his alarm and consternation (always signalled by the trademark blinking and mouth-breathing)!