This is just a short blip in a Vanity Fair video (from the time he was included in the Vanity Fair Hollywood issue) when they asked a number of celebrities who they would like to see play themselves in a film. 

Ben's answer starts about the 43 second mark.  I love how he compliments someone else's work before answering. 

Here's the link -
https://www.youtube.com/watch?v=9Lxox1WDJhQ

Just in case you couldn't hear it properly his answer is "Andrew Buchan" who if you look him up is a pretty good choice actually - right age, about Ben's height, trained in the theatre, has a facial shape like Ben (although I don't think he is nearly as handsome or interesting in his features) and seems to have acting "chops" as they say with some a fairly impressive CV. 

-Val