Well people have asked me what goes through my head when I design what I think characters look like from books. Honestly I usually have a really clear image already, but I do make small changes to them if there's something stupidly wrong that's completely obvious... ANYWAY, this is what went through my head when I designed Sirius Black at about the age of 16... I apologise a million and one times for my terrible writing (how come I can draw but not write?!) so this is what I wrote if you're fining it difficult to decipher:

Sirius is brave, loyal, reckless and cynical, his great redeeming quality, however, is how much affection he is capable of feeling even though he was shunned by his family. He’s good at spouting bits of personal philosophy, but he does not always live up to them. He has double standards just like everyone else and a terrible temper, that impish grin could turn into daggers bloody quick! Sirius has probably dated his share of girls... hell, his and five other guy’s, and he never knows when to quit. He's protective of those he cares about and will fight fiercely for them. He’s very bright, but a total troublemaker to the end...

As far as character design goes, Sirius’ animagus form (a giant black dog) is what is kept in mind most of all. His face has a dog-like quality that even I can’t put my finger on... but whatever it is, it works. Second to the doggishness, you also have to be mindful that his face needs to have the capability to become gaunt and very vampiric (even scary) to be the plausible face of the murderer on the Azkaban posters, hence the already visible cheekbones. And yet, he was described as havinf been a rather handsome young man before he was convicted.

Being one of the leaders of the Marauders, he has a mischievous air about him. You can usually see things like this about the eyes, so Sirius would have very smiley eyes, and J.K Rowling tells us that they’re grey, so that would add to the playful yet calculating mysteriousness. Ironically, Sirius is probably hardly ever serious if he could help it, therefore he would probably never just stand and stare at the ‘camera’ he would always be taking in his surroundings, looking for some fun, someone to play a prank on... and as for how he wears his clothes, he seems to be the type to not really care if he’s well presented or looks like he just rolled out of bed...