Yesterday, Beth talked about the small details that end up making a fictional - particularly science fiction - setting real. I heartily concur with her assessment. Small details make it easier to suspend disbelief.
In that vein - minor characters who are fully fleshed out serve to make your story richer, to enhance your main character's interactions, and give a feel of reality to your writing. I'm not talking secondary characters, or side-kicks - but those characters who make brief, yet memorable, appearances.
If you can embue your minor characters with backstory, reasons for action (or non-action), and interesting quirks - it's like filling your book's canvas with a veritable rainbow of color.
A few examples of minor characters from a variety of novels, movies & TV:
C3PO from Star Wars - he's prissy & fussy & incredibly loyal (okay - he's more than minor, but he's great!)
Scotty from Star Trek - he's no-nonsense, a bit stubborn, & an excellent engineer
Luna Lovegood from Harry Potter - can I just say how much I love Luna? She's a fabulous, quirky, & endearing girl
Mary Crawford from Mansfield Park - a woman far more interesting than even the novel's heroine.
Lots of fan fiction is written putting minor characters into starring roles.
What minor characters do you find well-written and would you like to know more about?