A waiver can occur in a number of ways: With respect to U.S. federal courts, Rule 601 of the Federal Rules of Evidence defers to state law the determination of whether a witness is competent to testify.
In other states, such as Illinois, the rule has been expanded to prevent an interested party from testifying about communications with a minor or a legally incompetent person.
In civil actions by or against executors, administrators, or guardians, in which judgment may be rendered for or against them as such, neither party shall be allowed to testify against the others as to any oral statement by the testator, intestate or ward, unless that testimony to the oral statement is corroborated or unless the witness is called at the trial to testify thereto by the opposite party; and, the provisions of this article shall extend to and include all actions by or against the heirs or legal representatives of a decedent based in whole or in part on such oral statement.
Except for the foregoing, a witness is not precluded from giving evidence of or concerning any transaction with, any conversations with, any admissions of, or statement by, a deceased or insane party or person merely because the witness is a party to the action or a person interested in the event thereof.
The trial court shall, in a proper case, where this rule prohibits an interested party or witness from testifying, instruct the jury that such person is not permitted by the law to give evidence relating to any oral statement by the deceased or ward unless the oral statement is corroborated or unless the party or witness is called at the trial by the opposite party.