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, professional conduct, 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, climb stairs, 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.