When I was young, my father always told me to treat everyone with respect. The example he always gave was going in for an interview and holding the door open for the janitor... because you never know who that janitor actually is. Well, if you come to my building for an interview in my organization, I will check with our building's receptionist to see how you walked in, how you checked in and how you interacted with others while you waited for me. Were you gracious? Did you say "please" and "thank you"? Did you hold the door open for others? Did you say smile and say hello to people? Our receptionist is an excellent judge or character and can quickly determine if someone won't be a good fit in the company.
Some time ago, I was walking into my building and ran into an obvious interview candidate in the parking lot. As we walked across the parking lot, I asked her if she had any problem finding a parking lot (I like to give people a reason to complain to see if they will). She said she did not and enjoyed the walk since it was such a nice morning. When we went separate ways she said, "Have a great day." and I wished her good luck on her interview. Of course, 10 minutes later, I came back downstairs and introduced myself and walked her up to my office. She did just as well in the interview as she did in the pre-interview.
My advice: be courteous and gracious to everyone, everyday, for no other reason than it makes life easier. As a side effect, it'll prevent you from doing something stupid like giving the middle finger to someone on the freeway only to find yourself sitting in his office in a job interview less than an hour later. True story. Maybe I'll tell it someday.