Must quote....

[windemere]

Some writers spend a long time exploring the off-page life of a major character such as Severus Snape, the potions master who nurtures a dislike of Harry. One story about Snape has stretched to 76,000 words.

The fact that actor Alan Rickman plays Snape in the films may have prompted many of the stories written about the potion maker.


http://news.bbc.co.uk/1/hi/technology/3004136.stm

Think the same could be said for Sirius? :D