Thin? Medium? Tall, short, dark, light? Bald? Ponytailed? Flexible? Stiff? Do they wear glasses or contacts or neither? Are their voices high, low, weak, strong?
Except in cases where it's germane to the story, or it's a major character, do you specify such physical characteristics? Do you think about them?
I tend not to, and I often think I should. A weakness in my writing is lack of physical description of people and places, with some rare exceptions. This may partly be inherent in writing plot-driven fiction. James Gunn, I think it was, said that science-fiction writers should avoid emphasis on characterization because of the nature of sf. (Or perhaps I'm - ho, ho - mischaracterizing what he said.) Certainly, characterization can be distracting when the plot should be moving along rapidly, with satisfying complications and resolutions. But at the same time, you want your characters to be real and the setting to seem real. It's a balancing act, and I often fear that I come down on the wrong side.
In movies and on TV, no one needs glasses, unless it's to show that the character is a nerdy scientist (knows everything about every field of science but nothing about the opposite sex, of course). No one has a hearing problem, unless it's a plot element. Nowadays, all the beautiful people, and many of ugly ones, come equipped with fearsome martial arts skills. Those are all conventions, but I find them distracting. Real and believable differences would help, even in action movies, and somehow one has to find the right balance so as to have such differences in prose fiction, as well.