I just watched M again, and as always, Peter Lorre is so magnificent. You want to make fun of him ... if you're of a certain age (like, for instance, MY age), you can do a Peter Lorre imitation, and he's mostly a joke. But he was a great actor, and in M, even when you know his big scene is coming, and even when you think you're going to laugh, once he starts in, you're taken in once again, with simultaneous pity and horror at this sick child murderer.
I read his bio on the IMDB, and found this anecdote especially good:
During the Hayes Commission investigation of 'reds' in Hollywood during the late 40s, Lorre was interviewed by investigators and asked to name anyone suspicious he had met since coming to the United States. Lorre responded with a list of everyone he knew.