Just like there are formalities as to how the witnesses must witness a valid Will, there are requirements that govern who may act as witnesses.

Generally

California Probate Code Section 6112(a) states: Any person generally competent to be a witness may act as a witness to a will.. Although, it is not a strict requirement that the witness be 18 years or older, it is strongly recommended that only adults act as witnesses to avoid later challenges that a witness lacked competence due to age.

Interested Witnesses

An interested witness is someone who stands to receive something from the Will. A Will is not invalid just because one of the witnesses was an "interested witness." That being said, unless there are at least two other witnesses who are disinterested, there is a presumption that the interested witness obtained the bequest by duress, fraud, or undue influence.

California Probate Code Section 6112(c) states:

Unless there are at least two other subscribing witnesses to the will who are disinterested witnesses, the fact that the will makes a devise to a subscribing witness creates a presumption that the witness procured the devise by duress, menace, fraud, or undue influence. This presumption is a presumption affecting the burden of proof. This presumption does not apply where the witness is a person to whom the devise is made solely in a fiduciary capacity.

Unless the presumption is rebutted, the interested witness receives no more than what he or she would have received if the decedent died intestate.

California Probate Code Section 6112(d) states:

 If a devise made by the will to an interested witness fails because the presumption established by subdivision (c) applies to the devise and the witness fails to rebut the presumption, the interested witness shall take such proportion of the devise made to the witness in the will as does not exceed the share of the estate which would be distributed to the witness if the will were not established. Nothing in this subdivision affects the law that applies where it is established that the witness procured a devise by duress, menace, fraud, or undue influence.

Lawyer or Hospital Worker as Witnesses

Estate planning lawyers frequently act as witnesses to their client's Wills. Having a lawyer act as a witness may be especially helpful since there are generally good public records on where he or she is located should there be a need for them later on. Hospital employees are permitted to ac as witnesses; however, some medical institutions may disallow this.

The witness requirements for a witnessed Will can be confusing as there are several requirements needed to create a valid Will. The following rules also apply to Codicils.

Number

Except for holographic Wills, every Will must be witnessed by at least two people. Indeed, some estate planning lawyers will arrange to have three witnesses during the signing of a Will. This may be especially helpful if a witness dies, becomes incapacitated, or cannot be located and there's a need to get the testimony of a witness who was living at the time the Will was signed.

Presence

Each of the witnesses must be present at the time the Testator signs his or her Will. Alternatively, the witnesses must be present while the Testator acknowledges that the signature on the Will is his. Although the witnesses don't have to sign the Will in front of the Testator or each other, it often does not make sense to do otherwise. 

Understand Document as Testator's Will

The witnesses must also understand that the document the Testator is signing is his or her Will. Sometimes, a lawyer may request that the Testator acknowledge in front of the witnesses that the document they are about to sign is his or her Will, just to make it abundantly clear to all who are present.

Notarization?

Notarizing a Will has no legal effect by itself. A Will must be witnessed by at least two people.

California recognizes 4 types of Wills:

1. Witnessed Will - A written Will that is signed by the Testator and witnessed by 2 people who also sign the Will. 
2. Holographic Will - A Will that is written in the Testator's own handwriting.
3. California Statutory Will - A form Will created by the California legislature that has "fill-in-the-blank" sections that the Testator can fill in. It must be witnessed by 2 people who also sign the Will.
4. Uniform International Will - A Will that conforms to the requirements of the Uniform International Wills Act. It must be witnessed by 2 people who also sign the Will.

Estate planning lawyers primarily work with witnessed Wills, as it is generally the most formal of the ones stated above. The requirements of a witnessed Will are outlined in California Probate Code Section 6110, which provides:

(a) Except as provided in this part, a will shall be in writing and satisfy the requirements of this section.

(b) The will shall be signed by one of the following:

(1) By the testator.

(2) In the testator’s name by some other person in the testator’s presence and by the testator’s direction.

(3) By a conservator pursuant to a court order to make a will under Section 2580.

(c) (1) Except as provided in paragraph (2), the will shall be witnessed by being signed, during the testator’s lifetime, by at least two persons each of whom (A) being present at the same time, witnessed either the signing of the will or the testator’s acknowledgment of the signature or of the will and (B) understand that the instrument they sign is the testator’s will.

(2) If a will was not executed in compliance with paragraph (1), the will shall be treated as if it was executed in compliance with that paragraph if the proponent of the will establishes by clear and convincing evidence that, at the time the testator signed the will, the testator intended the will to constitute the testator’s will.

As mentioned in part (b)(1) above, the Testator doesn't need to be the one who signs the Will so long as the person signing the Testator's name is doing so in the Testator's presence and at his or her direction. In addition, the witnesses must be disinterested parties and competent individuals. Finally, although the best practice is that the witnesses sign their names on the Will immediately after the Testator signs his or her name on the Will, part (c)(1) above, states that the witnesses need only sign their names on the will "during the testator's lifetime".

Because of the rules involved, estate planning lawyers are often hesitant to let the client personally oversee the signing of Wills and other estate planning documents. Best practice is to go to your estate planning lawyer's office so that the lawyer and the professional staff can ensure they are done correctly.

The primary purpose of a Will is to express the Testator's wishes with respect to the transfer of his or her property at the time of death. Every once in a while, a Will does not provide clear enough information to help the Executor or personal representative fully carry out the distribution of the assets of the estate.

For that reason California provides some helpful rules of construction. Here are a couple:

1. Preventing Intestacy: California Probate Code Section 21120 states that: "The words of an instrument are to receive an interpretation that will give every expression some effect, rather than one that will render any of the expressions inoperative. Preference is to be given to an interpretation of an instrument that will prevent intestacy or failure of a transfer, rather than one that will result in an intestacy or failure of a transfer."
2. Revocation of Gifts to Former Spouses: California Probate Code Sections 6122 and 6122.1 provide that gifts made to a former spouse or registered domestic partner are presumed to be revoked unless the Will states otherwise. The law assumes that you would not leave a gift to a former spouse or registered domestic partner if you divorced or ended your domestic partnership. 

A challenge of estate planning is drafting documents in a clear manner so that people in the future don't need a crystal ball to determine your intent. Along a similar vein, Wills and Trusts should be drafted in a way that capture reasonable contingencies that account for changes in your situation and that don't render your Will or Trust ineffective.

For example, you might suggest that a distribution be made to your "then living children" rather than a child who is alive at the time you create your Will. That way, if you have more children in the future, you may not need to amend your Will or Trust to account for later born children. Thinking through these possibilities is something that your estate planning lawyer should help you do.