When I sit at a table and a person is looking straight at me, I do not see her face. So I peer past her with both eyes to see her head and at least roughly recognize her face. The finer features of a face are not recognizable to me at least, at this distance. This means that I can not use faces to recognise people.
For example, the colour of the eyes is not useful information for me. I would have to approach someone very close to identify the colour of the eyes, most probably too close. And I would have to grab that person's head and turn it into a better light. So what are the identifying marks that I need? In the first place stands the voice. As soon as someone I've known for some time says something, I know who he is. Then comes the way to dress and move. Does someone like to wear loose clothing and move casually? Or are tight-fitting dresses preferred and a straight and economical way to move? Or a combination of both? The disadvantage of this method is that you have to have people around for a while to learn their characteristics in style and clothing. So I can confuse people several months after having got to know them, especially if we meet only once every two weeks. That is simply not enough time for me to learn their style. And to mess things up, it's enough to change a hairstyle to make me start the learning all over again.
Once I was with three or four people in the same room and it took me five minutes to realize that one of them is a friend i who I knew for a long time. What was different? Well, firstly, she wore her hair open, I knew her with a ponytail. Secondly, she wore a shirt that I had never seen before and somehow did not fit her style: Usually, she wears dark colours, this shirt was bright yellow. And finally, her voice was slightly horse as a result of a cold. I only recognised her after I listened to her talking for some minutes, then she used a certain phrasing and tone, that I could associate with her.