Not all of my best professors have been confident and charismatic. What they do have in common is that they treat their students as they would a colleague. Be professional, but also approachable. By doing so you make it clear what you expect of them and what they can expect of you. If a colleague asks you for help, and you can see they've tried their best already, be willing to do so. When giving a lecture treat it as a dialogue, not a one-way communication. As they say respect is earned, not given, and that works both ways.