Sketching a random character is a lot like working at a bar.
You don't know who you'll be serving until they walk in, and you'll likely know little to nothing about them.
Maybe they like martinis, shaken, or possibly stirred. They could like scotch, whisky, wine, sake, who knows?
What's important is that you serve them, and that requires attention to detail.
Every, small little thing accounted for.
The man before you has visible muscles, there's a couple of tattoos, a scar or two.
He probably worked in the army, meaning that it'd be best to get his order done fast. Straight to business.

Perhaps the strangest part of this metaphor are the regulars.
They change, without you even realizing it. Maybe a new haircut, some better glasses.
What I find odd is that a nameless character that appears often is allowed to exist in the corner of my mind, living where I can't see them. I know they're there.
Mystery regulars, that come in, drink, and leave are perhaps the strangest of them all.

So, do you guys ever have a similar... phenomenon?
Characters that only exist on the tip of your pencil?
What can I do to make them exist? To make them a character?
A character requires two major things; A personality, and a goal.
While this I can understand why, I cannot comprehend how.