A persuasive witness makes effective use of voice and gestures.

Studies verify experts are most believable when they:

are perceived as calm and composed in their presenting style without anxious verbal and nonverbal cues; 

speak in language the jury understands--without resorting to hypercorrect and less personal manners of speech (eg., "the client"); and, 

testify in the way that best presents the story--including voice modulation, facial expressions, eye contact, and gestures appropriate to the narrative--and in a manner that conveys confidence and credibility. 

See Robert J. Cramer MA, Stanley L. Brodsky PhD, and Jamie DeCoster PhD, "Expert Witness Confidence and Juror Personality:  Their Impact on Credibility and Persuasion in the Courtroom" in J Am Acad Psychiatry Law, 37:63-74 (2009)

Make sure your testimony has its greatest persuasive impact by:

holding the interest of your audience--the jury or the court--by organizing it so the listener can follow easily;

speaking the testimony with a voice pleasing to the ear of the listener--the jury or court; and,

conveying the testimony clearly and simply--while ideally connecting with the theme of the case.

As a lawyer, Harry can also teach you how best to prepare and consult with the lawyer:

early involvement to help with the case strengths and weaknesses;

incorporating the attorney's theory of the case;

educating the attorney;

collaborating with attorney to create questions for you and other witnesses; and,

how you can best get off the witness stand to interact with the jury.

The techniques Harry teaches are not new.

More than two thousand years ago Cicero--the Roman philosopher and orator, set forth classic rhetoric principles that are still essential elements to storytelling today: 

what to say; 

organization of what you say;

the language you use;

memorization of the presentation (not necessarily the speech); and,

delivery of the presentation.

Witnesses may be experts, appearing as part of professional duties, or testifying to other facts within their personal knowledge.  Like other public speakers, witnesses can suffer the same glossophobia (fear of speaking) afflicting many people.

If you speak well, these techniques will make you better.  When you combine a good voice--with an understanding of how to use gestures, eye contact, and body language consistent with your vocal message--you will be a more persuasive witness.

Mastering your voice and body is not a simple task. It takes practice and a willingness to try new things that will make you a better speaker, presenter, and witness.