What is the good faith requirement for a witness to be found "unavailable as a witness" at a hearing?

The good faith requirement is codified under California law in Evidence Code section 240, which provides that a hearsay declarant may be found " 'unavailable as a witness' " when he or she is "[a]bsent from the hearing and the proponent of his or her statement has exercised reasonable diligence but has been unable to procure his or her attendance by the court's process." (Evid. Code, 240, subd. (a)(5).) Courts often describe the reasonable diligence requirement as one of " 'due diligence.' " (People v. Cromer, supra, 24 Cal.4th at p. 898.)