Being a trial lawyer is truly a theatrical experience. It involves not only the technical elements of theater, like staging and voice, but also the truly "act-y" parts. What will you say, and when? How will you react to a question you already know the answer to? And most importantly, how do you impress a jury?
Add to that your own witnesses, or even your own client. They may need to step up their acting game as well. Should all of you take acting classes? Here are a few points to consider:
Lawyers, at least, should take acting classes to learn the technical parts of acting. For example, how do you project your voice? The jury needs to be able to hear you, and they can't do it if you're mumbling. You should also practice blocking, which is where people stand and where they move in a performance. It's important that you stand in the right place so that the jury can see the witness -- or, in the case of cross examination, so that the jury can see you.
As far as jurors go, they want to hear a story, so you've got to come up with one. Acting classes can teach you how to present a compelling story. "Story" doesn't mean you're lying, of course. It just means that you're presenting your theory of the case in a way that piques and retains the jury's interest.
Witnesses Need Help, Too
Oscar Pistorius got flack when the press learned he'd taken acting classes, but the point of that isn't -- as the inference was -- to teach him how to lie to the jury. No, the point is to make his statements seem more convincing. Juries aren't just evaluating what a witness says; they're also evaluating how the witness says it.
Even a person who's completely telling the truth can come off as shifty if he doesn't know how to appear that he's telling the truth. (For example, a person who doesn't say things clearly and with confidence can look like he's hiding something.)
Acting Goes Beyond Trials
Learning basic acting skills can help in depositions, or in any other position where you have to talk to others. Improvisation is also helpful, as it trains you to be able to think quickly.
Ultimately, acting classes are a good idea because they increase your public speaking confidence. Not every trial lawyer is an extrovert who craves attention and can control a room. In people for whom that doesn't come naturally, those skills need to be taught, and teaching others how to have a presence is something acting teachers do every day.