Theater, Going to
I tell clients that a trial is part perception management.
I give my clients Rules of Engagement for their conduct in and out of the courtroom.
If I must paraphrase Shakespeare's "As You Like It," I'd say that every courtroom is a stage and we the players.
Why actually is it important to consider how you act on the stage.
Consider: Laurie L. Levenson, Courtroom Demeanor: The Theater of the Courtroom. Legal Studies paper No. 2007-30 July 2007, at 27.  The article focuses on the impact of a defendant’s appearance and demeanor on jurors, and perhaps also the judge. People believe, rightly or wrongly, that how you act or how you say something indicates truthfulness, mendacity, or a consciousness of guilt. Professor Levenson's article should put doubt in your mind about the value of demeanor.
Based on interviews with jurors in high-profile cases, it is undeniable that a defendant’s demeanor and appearance in the courtroom can influence their decisions. Although demeanor impacts both high-profile and routine cases, the high visibility cases provide the starkest evidence that jurors readily consider all conduct in the courtroom, not just the testimony of witnesses, in reaching their decision.
At p. 7.
Rather, as we have learned from psychologists and sociologists, there is a dynamic to the courtroom that is more akin to, but not precisely like, a theater. Jurors use all of their senses, including their intuition, to reach their verdicts.108 “In the courtroom, nonverbal communication subtly affects the entire proceedings of a trial.”109 Yet, because courtrooms are not for mere entertainment or education, we expect that the verdict in the courtroom will be based on concrete, verifiable information and not impressions of the parties’ personalities.
At p. 30.
Some other discussions can be found in:
Lawson Hennick, Was it something I said? Non-verbal cues in court. Ontario Trial Lawyers Assoc., 2 March 2017.
J. L. Hoffman and A. Weiner, The Juror as Audience: The Impact of Non-Verbal Communication at Trial. 32(3) OREGON ST. BAR 1 (2013).
 This paper can be downloaded without charge from the Social Science Research Network (SSRN) electronic library at: http://ssrn.com/abstract=999471.