Start Like Attila the Hun, Finish as Mr. Rogers
Everyone has a personality. Some people are affable and informal. Others are distant and reserved. However, whatever one's core personality, most people have a range of personality expressions, depending on the time, place, circumstance, and surroundings. Whatever your teaching personality, it will be easiest for both you and your class if you start out the semester at the most formal extreme of your personality, and then if things seem to be working out okay, you can relax a little. If you start out as a "tough guy," and find after a time that the class is doing well and working hard, if you ease up a bit, the class will breath a sigh of relief that you are now showing some human qualities. On the other hand, if you start out cozy and friendly (Don't call me Professor Farnsworth, just call me Steve), and the class gets the idea that you aren't really serious about things like deadlines, if you get tough later on, they will feel like you have turned against them, and aren't really as nice as you seemed to be.
Your style of dress will give the class rich clues as to the kind of personality you want to project. I always start the semester with my best suit, a power tie, and the gold pocket watch my Dad gave me. Then, as the semester wears on, if things are going well, I shift to a sport jacket, eventually lose the tie, and may even wear a sport shirt. On the other hand, if I feel they've been lazy in preparing for an exam, and need a bit of talking to about this problem, back comes the suit. If I feel that they're starting to become overwhelmed and need some reassurance, I shoo the moths off my comfortable ol' cardigan sweater, and do everything except change into slippers at the beginning of lecture. However, the decline of professional dress standards (or elimination of archaic dress codes, depending on your point of view) means that "formal" is now definitely a relative term. "Formal" in some quarters means you wear a shirt over your Tee.
The personality you project will have a lot to do with the day-to-day behavior of your class. High school students are used to the idea of "testing" their teachers to find out what they can get away with in class, and then adjusting their behavior depending on the response. In recent years, they seem to be bringing that habit with them to college. Many of my colleagues are now complaining about a dramatic increase in rude behavior in freshman classes, and they are not quite sure what to do about it. I rarely seem to have this problem, and I think one of the reasons is the "tone" I establish on the first day of class. I try to suggest to them that they are in the intellectual equivalent of Marine recruit training, I am their drill instructor, and if they wish to find out if I really mean these things I am saying about the course, they are welcome to try me out. This approach is not for everyone, of course, but it works just fine for me. Later on in the semester, the drill instructor gets sent to the showers, and is replaced by Mr. Wizard, Bill Nye the Science Guy, Beakman, or the appropriate, affable TV science teacher for your generation. Every once in a while, however, the students get a hint that the drill instructor is waiting in the wings if they don't study, and they better not do anything to bring him back, because he's out of control. By the end of the semester, as they're getting ready for finals, I'm all tea and sympathy because the high school bravado has pretty much disappeared, and they really do need a lot of reassurance and confidence building. Going from tough to nice is appropriate given the changing needs of the students during the semester, but going the other way seems to produce student ill-will at an awkward time--student evaluations of faculty---.
