You may have more of a physical response. You impression may cause you to seek eye contact, to cross the street and distance yourself, or to walk past them and try not to attract attention.
How we react to what we see depends greatly on a myriad of factors: how we were raised, where we come from, what we believe, what we're comfortable with, past experiences, and personal preferences to name a few.
In our writing we have a lot of perspectives to consider when it comes to first impressions. Of course we need to pay attention to our POV character(s). What they think of people and the factors that influence how they think and act will steer the course of the story.
But it is also very important to consider the other character(s) in the book who are seen by our POV character. Her impression of herself will, after all, influence how she acts and reacts. Her appearance will also be influenced by something. Maybe she's allergic to make-up, or maybe she knows she looks better with long hair. Maybe there's a story there. Was she just in a fight? Did she grow-up without really learning how to take care of herself? Did she only ever get attention from her looks or for her athleticism? Probing these types of things helps us uncover a greater depth to the characters involved in the story, as they influence the impressions the character has of the POV character(s) and how they interact with each other.
There are two other perspectives that are important to take into account, those of the reader and of the author, in other words, you. When you describe a character with blond hair, do you automatically think she's going to be ditzy? Does your reader? What are the stereotypes that will unconsciously be drawn from in forming a first impression of this person? As the author you can help steer your reader's perspective closer to or farther from some of the common alleys their first impressions may try to lead them down. But at the same time, as authors we need to be aware of stereotype traps that we may fall in to. If we find ourselves in one, it's better to have done so consciously and with a storytelling purpose.
What got me to this subject? I was reading in Smithsonian Magazine and there was a really good article about tattoos and scarification. (Looking at the World of Tattoos by Abigail Tucker. Smithsonian Magazine, October 2010.) I don't often think about the depth of meaning tattoos and scars can have, so I enjoyed the way the article brought it out. I realized it somewhere at the back of my mind. Realized it when friends told me about their tattoos and even some of their scars. But I had just left them as loose thoughts swirling at the back of my mind. This article grabbed the threads and tied them together as I read.
Whether personal, social, ritualistic, or cultural, or accidental - any markings we have on our bodies tell something about us. About who we are and where we came from. Even our clothing - the things we choose to, or out of necessity need to, cover ourselves with tell a similar story. It's a like an onion growing around us. Everything we've surrounded ourselves with helps to tell the story that is uniquely us.