Enforced Method Acting: According to Hugh in a later interview, Stephen Fry had never been able to convincingly fake hitting someone. So when the script called for him to hit Hugh - which happened quite often - he would actually hit him. So that wincing and those cries of pain you hear from Hugh in this show are mostly real.
A lot of the time it's accidental though. A good example of this is in the racing driver sketch. Near the end you can see Stephen's eyes widen in shock, before he folds his arms and looks away.
Fake American: Hugh Laurie plays an American country singer in one sketch, and he and Stephen play American soldiers saying "ass" a lot in another.