Hi Dana, thanks for sharing these insights - I completely agree. I also want to add two things: 

1 - If you do your preperation well, your confindence will build. And if you're convinced that the job you're applying for is THE job for you - it will help you to remain calm during the interview and don't get into the squeaky voice mode.  

2 - If you're really unsure about your posture and are not certain you will make it through the interview, take it one step further during preperations: tape yourself while practicing an interview or even just a conversation. 
Look at yourself and see what your body language is. Watch the tape with the person you had the conversation with to hear someone else's opinion about your presence, your tone of voice, volume, etc. 
It will help you to come across as a balanced job seeker and it is easily done with everyone having a camera on their phone. 

Good luck! And best regards, -itske