Why do interviewers respond positively to assertive body language? How can a job candidate ...

It demonstrates a can-do attitude and confidence. When you meet the interviewer, you can give a firm handshake and make eye contact. Walk briskly. Sit, stand, and walk with your head up and your back straight. Aim for a pleasant, uplifted facial expression. Occasionally nod your head and gesture to convey agreement and emphasis. Avoid frowning, clenching your jaw, and making other negative expressions.

Dont fidget. Watch the interviewers body language. The interviewer may lean forward, signaling you to expand on what you are saying. If the interviewer shuffles papers, looks around the room, or gives other nonverbal cues that you should finish speaking, heed the signal. Continue listening and watching to determine whether what you are saying is clearly understood. Retreat from a subject if you observe that its not being well received.
Subtly mirror the interviewers communication behaviors.

I sincerely appreciate this help!