I saw this over at Apollos Academy, and I had to share it.
Apparently, it's from a British sitcom entitled "Extras", about a professional extra who's trying to make it big. It appears that they get a lot of big-name actors on for little bit parts, playing themselves. And it appears that a lot of these clips show up at Youtube--we've now seen the one for Patrick Stewart, and for Kate Winslet, and a couple for Robert DeNiro. I also saw a longer one for Daniel Radcliffe, although I didn't watch it. Be forewarned if you want to go browsing these clips: most of them are loaded with sexual humor.
Anyway, here's Ian McKellan, explaining what it means to be an actor:
(And there's no sexual humor in this particular one, in case that's important to you.)
And in case you're interested, here's the one of Patrick Stewart. (And there is sexual humor in this particular one, in case that's important to you.)