The wikipedia article point out that M is one of the earliest films to give a character a music theme; "In the Hall of the Mountain King" plays in association with the killer's presence, to the extent that it signals to the audience when he's out of shot but nearby. A famous, modern version of this technique, called Leitmotif, is the Star Wars Imperial March theme that mirrors Darth Vader and the Dark Side on the screen and in the minds of viewers.

http://en.wikipedia.org/wiki/M_%28film%29
http://en.wikipedia.org/wiki/Leitmotif