I always forget my character’s eye colour.
I can just about remember hair colour.
I often forget something vital about their personality.
In other words, I have great difficulty remembering all the little details about all my characters. (Apparently, contrary to what I imagined, George R.R. Martin keeps it all in his head. I don’t know how. He must have a brain the size of a planet.)
I’m not the only one who forgets the details. Arthur Conan Doyle referred to Sherlock Holmes’s housekeeper as Mrs Hudson in every story but one, when he referred to her as Mrs Turner. And Dr Watson’s war wound kept migrating from his shoulder to his leg.
So, I bought an address book (although I had to go several different places to find one. Apparently there just isn’t the demand any more). A nice big thick one. Every character has an entry with name, physical appearance, salient facts and a few terse notes on character. Just enough to jog my memory. I just have to remember to enter people as I create them.
Now I just have to careful not to lose my character guide.