The general rule is that you need an expert witness to testify when the subject is “sufficiently beyond common experience that the opinion of that expert would assist the trier of fact.” Evid C §801(a). When it comes to a person’s sanity, you don’t always need an expert.
As Joseph DeAngelo (the “East Area Rapist”) may soon find out, a divorced spouse can spill a lot of beans. He was separated from his wife for years, but she’s just made divorce official and that’s a boon for the prosecution in his upcoming trial. But is she still limited in what she can say?
Before your friendly witness is called to the stand, you should go over some general guidelines with him or her. This is particularly true of inexperienced witnesses, but it can’t hurt to review these admonitions even with someone who’s very experienced in giving trial testimony. You also might want to give them a hard copy to look over just before testifying.
In civil cases, you can call an adverse party or witness in your own case. Evid C §776(a). But just because you can doesn’t mean you should. Before deciding to call an adverse party or witness, definitely check out this chart laying out the strategy considerations—reasons to do it and reasons to steer clear.