Most states require that two witnesses subscribe to the execution of a prepared will. Since the testator is deceased, he cannot verify his signature nor testify as to his intent, so the witnesses testify in his place. Generally, witnesses must swear to the testator's identity -- that the person who signed the will was who he claimed to be -- that he was of legal age, possessed the required testamentary capacity, executed the will of his own free will and had full knowledge of the contents of the will. As of November 2010, no state requires that the testator's signature be notarized.
Witnesses must be 18 years or older and of sound mind. The law prefers disinterested witnesses -- witnesses who are not also heirs under the will -- and the majority of states specifically require them. In those states, a witness who also stands to inherit under the will may lose the inheritance. While a notary can serve as a witness to a will, she will sign as a disinterested witness, not as a notary.
Once a will is admitted into probate, the signature must be proved to be that of the deceased. The subscribing witnesses generally must testify in probate court. The parties can avoid this inconvenience if a notary is present at the time the will is executed. The testator and witnesses sign the will and acknowledge their signatures in the presence of a notary public who affirms their signatures. If this procedure is followed, the will is termed "self-proving" and no other testimony is required to prove the will.
Holographic wills, or handwritten wills, are valid in many states, including California. Holographic wills by definition cannot be preprinted, nor should printed materials be incorporated into them. These wills do not require affirming witnesses or need the testator's signature be notarized. According to the National Notary Association, some holographic wills are invalidated by notarization.