Apparently during casting for House, the producer, Bryan Singer, was despairing of finding an actor who could nailt he required level of sarcasm
After getting the job, it wasn't until Hugh Laurie lapsed into his natural voice during a break in shooting that anyone on the crew realised he was English
(Darth Foxtrot A one-man army dedicated to making fetch happen, Tue 15 Feb 2011, 16:44, 1 reply, 15 years ago)

I can believe that.
See also an example closer to home: I am led to believe that Richard Coyle was halfway through filming the second series of Coupling before the producers realised he wasn't actually Welsh.
(LongJohnBaldry, Tue 15 Feb 2011, 16:47, Reply)

And it wasn't until the eight series of Red Dwarf...
...that they realised Craig Charles is a noncey, talentless cunt. Isn't showbiz funny?
(Tugnut Ex of this parish, Tue 15 Feb 2011, 16:48, Reply)

he's not Welsh?

( Vipros. clever got me this far, then tricky got me in, Tue 15 Feb 2011, 16:48, Reply)

He was english in Going Postal

( Agnostic Antichrist Baltimora, Tue 15 Feb 2011, 17:06, Reply)

Jeff was a superb character

( Agnostic Antichrist Baltimora, Tue 15 Feb 2011, 17:06, Reply)