I'm always shocked when I discover that an actor has a different accent than I heard them use in a film or TV show.
Erik Singer would be less surprised by such a realization. Singer is a dialect coach, which means his job is to work with actors when they must adopt an accent that isn't the way they actually speak. Singer sat down to shoot a 16-minute video where he takes a quick look at numerous famous and infamous performances by actors assuming accents other than their own. Over the course of the video, I learned a lot more words and terms about the art of speaking than I've ever known.
Singer has no problem pointing out when an actor's accent is off the mark. He also sweeps in to point out small issues in the performances that showed perhaps the actor didn't have enough time to prepare. I'll say this: he really thinks Daniel Day Lewis is incredible at his accents. I also love Lewis, so I approve of his opinion here. In the end, Singer points out that it is rarely the actor's fault if their accent is sub-standard. He blames a combination of the production, the director, and the dialect coach. If a performer is not given enough time or proper support in learning an all-new way to speak, how could we expect them to be any good at it?