In your process of interacting with prospective employers, your conduct will be evaluated and can be a make-or-break factor in whether you move forward in the employer's candidate screening process. Good manners, appropriate attire, and your non-verbal body language will be judged.
Be prepared, and practice the skills you need for success. Practice handshakes. Give your interview attire a trial run to be sure you can walk, sit, and move comfortably. Practice good table table manners routinely. The more you practice, so that a professional demeanor is natural to you, the more you can focus your energies on listening to your interviewers and responding appropriately (and the less you'll need to think about whether your demeanor is appropriate).