They play, and they play very well, you really believe and empathize with them. Dean-Charles Chapman and George MacKay perfectly reveal their characters, in fact, one of the main advantages of the film is the actors.
I didn’t understand the situation with Benedict Cumberbatch a little, because such a cool actor was put at the end, in one short scene for 5-6 minutes. Although the moment when he appears in the frame looks very good (apparently this was the calculation).