Apparently Mr Murray - though famous for this - isn't unique. I was listening today to a fascinating programme on emotions. At one point, someone mentioned that we tend to think we can read emotions well in faces, but actually the situation is more complex than we think. He cited a study by someone who photographed sportspeople at the point where they won or lost very important matches, and asked people to say whether the people in the photographs were happy or sad. Apparently, people couldn't work out which were which. So it's not just Mr Murray.
It doesn't surprise me. Stephen Fry was saying on QI that when actors want to express and emotion that has already been implied they often keep their faces neutral. We, the audience infer and do the work for them.