Music is a good shorthand way to delineate character. If you want to tell the reader a lot about a character in a small space, just tell them what their musical taste is. You'll get their age, their background, whether they're gregarious or a loner.
The most difficult part of any crime novel is the plotting. It all begins simply enough, but soon you're dealing with a multitude of linked characters, strands, themes and red herrings - and you need to try to control these unruly elements and weave them into a pattern.