Sometimes I don't speak loud enough. People say to speak through/with the diaphragm - how do I do that and how do I know if I am doing it right?
The motions and body movement occasionally seem stiff or not natural. Is it over exaggeration? Lack of being in character? Not relaxed? How can I fix it?
Good questions... A trick to help you be louder is to tighten your stomach. Try tightening your stomach and see if it helps you speak louder. It's hard to do at first but if you do it enough it will happen more naturally.
Being stiff and not natural has different causes. If your director has strict blocking, you might be thinking too much about that. Something that helps is to figure out why you are moving. If you give yourself a reason to move then it might feel more natural. Are you moving toward another character because you want their attention? Are you moving away because you are scared? Think about the reason for the movement for your character (and not just because the director said to do it). I hope that helps!