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.
After a party objects to the admission of evidence, the proponent of the evidence can respond by arguing that the objection doesn’t apply or isn’t valid, or that the evidence is admissible because of an exception to the ground stated in the objection. Where that argument is made can make a big difference—should you argue in open court or in a sidebar conference?
As with all witnesses, you must be able to control an expert witness during cross-examination. But many experts with experience in testifying treat cross-examiners like presidential candidates deal with the press: they ignore the question asked and answer the question they prefer. Here’s how to keep experts under your control.