Sort-of continuing on from my last, rather slapdash post...
As has been noted by plenty of writers, you always wind up knowing more about your characters than makes the page. And as I have noted here and there in the past, I always end up knowing a lot. So much so that the characters may as well be people I live with. Full names, dates of birth, maybe not their entire life story but a very good chunk of it up to the here and now, even an idea of what their mobile phone and cars look like.
I stated in a recent post that the Addie (protagonist of recent WIP) is a closeted lesbian. Whether or not this actually comes up remains to be seen, but as of this moment I'm thinking it won't actually actively be raised in the story. At best, hinted at. So if it's not coming up in the story itself, why is it important? Because that's who she is. Take everything else the same and make her heterosexual and she's not Adeline Carrow. Each and every piece of information helps me to write her more accurately.
(There's also the added fact that her being straight rather diminishes the point of another character, but that's another ramble for another time)
The same is true of the fact that Jack Walker smokes a very particular brand of cigarette (Gitanes Brunes) and carries a modified Makarov PM. Or of the fact that Ashley is allergic to peanuts. Or that Addie's car is silver.
To know their clothes and their tastes and their personality is to write them. These characters are not mine. They are people, just the same as you and I and the Eastern European guy who lives across that street and I swear is a mafioso or a spy. The only way you can ever hope to do them justice is to know them just as well as you know yourself, or your partner, or a dear friend.
To know a character is the only way to write them. To know their story is the only way to write it.
That's why I like knowing everything about a character, or as near to everything as you could ever hope to know about someone. Otherwise, I wouldn't be doing them justice. And if I can't do them justice, who am I to try and tell their story?