When he wasn't acting in films, he also did radio, audio books and was also a narrator (I first heard his voice before I ever saw him act in Cosgrove Hall's The Wind in the Willows). He was also a major television star, playing Bertie Wooster before Hugh Laurie did and fought with the BBC for years in order to bring Lord Peter Wimsey (his best role) to the small screen. I'm actually halfway through reading his autobiography at the moment and my impression is that he was as nice, charming and amusing in person as he was on the screen.
If you've never seen one of his films before or haven't seen one in years, then might I recommend my two favourites: I'm Alright Jack and School for Scoundrels.
Here is a clip from School for Scoundrels were Mr Carmichael and Alistair Sims demonstrate how to One Up a couple of bounders.
And of course, the famous tennis match.