Paterson Joseph's accent.
He's defitnely a good actor, but his attempt at an American accent in Jekyll was pretty awful. I'm not sure who he was patterning himself after, but I've never heard an American speak in such away. Some of it sounded like he was trying to be from the deep South in the 1940's and some of it sounded like he was from the modern era centeral Eastern seaboard. Whatever it was, it wasn't good.
If he was an American trying to play a Brit; it would be like an actor delivering 1/2 his lines in a Yorkshire accent, and the other 1/2 of his lines in a South London accent and just figuring he was using a "British accent" the whole time. Either way it sounds fake and just doesn't make sense.