In real life, people tend to be almost "type cast." Know what I mean? Like the ER nurse I saw the other day, who looked like he probably lifeguarded as a teen--he was tall, nice looking (although my husband is much better looking--just throwing that out there in case he reads this. Haha.), and very much gave the impression that the situation was under control. My husband and I actually laughed on the way out of the ER (everyone seems to be fine, by the way) because he looked SO MUCH like an ER nurse who should have been in some medical drama.
But seriously, this happens in life all the time. People LOOK like the kind of people they are, or like the job they have, very often.
When was the last time you walked into a library and the librarian was a 6'4'' man with broad shoulders and a young-Harrison-Ford-as-Han-Solo smile?
We make generalizations about people for a reason. Life experience has taught us that it's often right. Now, let's translate this to stories. What this means for me in my writing is that I know I have to make characters look a certain way for a reason. I'm not saying my character has to look stereotypical, by any means. Let's look at our librarian character, okay? You don't have to make the librarian a woman with mousy brown hair (said by a woman with hair about that color so I'm not hatin' okay, y'all? Haha.), who's petite with thick glasses, and always wears cardigans buttoned all the way up. If you want to, that will resonate with people. They'll think "Yes! Librarian!" And that's okay. I'm not arguing for stock characters, per se, not if they're important to your story, but sometimes people look like we'd expect and they should in our fiction too.
But what about the opposite? I think it's MORE than okay too! Say you want your librarian to be a 6'4'' man with broad shoulders and a young-Harrison-Ford-as-Han-Solo smile. That's totally fine, but let's face it, you've going to have to address that. Some character is going to make a comment about how he does not look like a typical librarian. There's going to have to be a reason he became a librarian instead of a football player, or police officer, or soldier. See what I mean?
Basically it breaks down this way. How your characters look matters. They almost (over-simplifying this a little) need to look just like their personality/job dictates, or totally opposite. Or even if you don't find that to be true, they need to have their looks for a reason. Don't give everyone blue eyes just because you like them. Don't make your hero look just like your favorite actor because you like how he looks even if it doesn't fit the character, okay? =)
How do you pick how characters look? Do you think you think through their physical descriptions consciously, or do they just "end up" looking like they're supposed to? Do you find pictures? Draw pictures?