Actors Changing Characters

I recently read an interview with Stephen Falk commenting about the second season of You're The Worst and he commented that Chris Geere's character of Jimmy was not intended to be British but the character was specifically altered after seeing his Audition. This surprised me as a Brit because a lot of Jimmy's humour is so quintessentially British that I thought his character would have had that running through all of the early scripts. So an interesting article could be how much a certain actor can influence changes in their character, a few more examples springing to mind are Morgan Freeman in Shawshank and Mackenzie Crook in The Office. Could be Film or TV.