California Code, Welfare and Institutions Code - WIC § 15657.7

Sec. 2. (a) All wills except nuncupative wills shall be executed in writing.

(b) Any person competent at the time of attestation to be a witness generally in this state may act as an attesting witness to the execution of a will and his subsequent incompetency shall not prevent the probate thereof.

(c) If any person shall be a subscribing witness to the execution of any will in which any interest is passed to him, and such will cannot be proved without his testimony or proof of his signature thereto as a witness, such will shall be void only as to him and persons claiming under him, and he shall be compelled to testify respecting the execution of such will as if no such interest had been passed to him;  but if he would have been entitled to a distributive share of the testator's estate except for such will, then so much of said estate as said witness would have been thus entitled to, not exceeding the value of such interest passed to him by such will, shall be saved to him.

(d) No attesting witness is interested unless the will gives to him some personal and beneficial interest.  The fact that a person is named in the will as executor, trustee, or guardian, or as counsel for the estate, personal representative, trustee or guardian does not make him an interested person.