The JSAC takes the form of role play exercises. My best advice is to read all the information they give you carefully, then listen to what is being said, as the role player will be trying to get information over to you. If they repeat something, it is because it is important, and you have not picked up on it. They will not repeat it again!
Be yourself, remain calm, remember that if the role player shouts at you, (and they will) it is not directed at you personally, but more a sign of their anger and frustration at a situation. Let them get it out of their system, but don't allow them to swear and be rude to you, say something along the lines of 'I am not swearing at you, please don't use that kind of language to me', but don't let it develop into an argument. Once you have explained the situation, they will calm down and listen to your point of view.
I know that it is a stressful situation, but speaking as someone who has acted as a role player in JSAC's, (and nearly been attacked by candidates) I can tell you that if you listen carefully, all the clues are there.
Good luck.