Holmes stories are somewhat different in that they may legitimately be said to be character studies as well as mysteries. Most of the Holmes stories are narrated by Watson, after all. The two aspects, mystery and character study, leave little room for characters aside from Holmes and Watson to really develop. The only time when the villains/perpetrators really come into their own is when their actions drive the current plot, which is comparatively seldom; Holmes usually solves crimes that have already happened.