At the time the will is executed the witness must be mature enough and of sufficient mental capacity to understand and appreciate the nature of the act she is witnessing. Her attestation must be of a level to which she may testify in court, if need be, to her act of signing a will and witnessing the testator's signature. The competency of a witness to the signing of a will is determined at the time the will is executed.
Under the minority common law, an interested witness to the signing of a will is not a competent witness. If there are only two attesting witnesses, and one is interested, the will will be denied probate. The majority is contra. Under majority law today the gift to the interested witness is void, but the will may still be probated. Under the Uniform Probate Code neither the will nor any of its provisions are affected by the fact that an attesting witness is also a beneficiary under the will. Under the supernumerary exception to the interested witness rule if there are more than enough disinterested witnesses to prove the will, the interested witness rule is not applied.