Perhaps you have become involved in some way with the will of someone who lived in California but who drafted and signed the will in another state. Before you proceed with anything, your top question may be, "Is this will from out of state legal to use in California?"
The answer is generally yes, but there can be exceptions. As with many things, prevention is the best cure. If you are moving to a new state, you should have your will done or updated in that state.
California generally says that your will is valid if you sign it and two uninterested witnesses also sign it, among other requirements. (You can also handwrite your will, called a holographic will, and sign it with no witnesses needed.)
If the out-of-state will has no statement saying that the witnesses swear to having been present to watch the person sign and that he or she was mentally competent to do so and under no duress, these witnesses may need to be found. If you cannot locate them or they have died, that could present a problem. This kind of will is a self-proving will, and they can help the probate process go much more smoothly in California.
Near the ends of their lives, many people are taken care of by caregivers and want to recognize them with bequests in their wills. However, California law, for the most part, requires that an independent attorney should review any will that contains this provision. If the person leaving the will came from another state and has such a bequest, this part of the will may not hold up under California law. Thus, a very important wish of a deceased person might not be carried out.
The same principle applies to fiduciaries of the person writing the will too, not just his or her caregivers.
If you do not properly make or sign your will under the laws of whatever state you wrote it in, then it may be invalid in California as well.