The most common thing here is that someone is trying to describe someone else so that you can describe that person to a third party. My experience is that doesn't tend to work very well, unless you know they're wearing a distinctive jacket or something on that particular day. I tend to ask people to text me a photo that I can then show to other people.

Depends on context. Am I looking for someone? Am I trying to remember what someone else thinks they look like?

It also depends on the context of the person. Like, an actress in a TV show, everything from how he's dressed to how she holds herself to their facial expressions to whom was cast are likely to be intentional, made to contribute something to the understanding of the character they're playing. When it comes to a regular person on the street though, what's the important thing to know about them? I probably don't care that the man downtown dressed in pajamas and playing a tuba has brown hair, or what race he seems to be. The unusual things are probably why the picture (if you're describing a picture) was taken, or if we're together walking past this guy, those are probably the things you noticed about him, so they're the things to mention.