Going through literature, I see a wide variety of approaches to the description of the characters' physical appearances -- the cut of their face, their build, the clothes they wear. Sometimes I have read that such descriptions help the reader picture the character better. Obviously this is important if the physical characteristics are relevant to the story: a sexy girl might use her wiles in a spy story, or a diplomat would be elegantly dressed, or Sherlock Holmes could tell the past of a character by his shoes, etc. But if it is not really relevant to the story, I feel sort of funny making up random physical descriptions. How much physical description is advisable, and under what conditions?