The court, upon motion and sufficient cause shown by either party, may grant a postponement of the hearing or trial. A motion by the defendant for a postponement on account of the absence of evidence may be made only upon affidavit showing the materiality of the evidence expected to be obtained, and that due diligence has been used to obtain it. If the motion is based on the absence of a witness, the affidavit must show what facts the affiant believes the witness will prove, and not merely the effect of such facts in evidence, and that the affiant believes them to be true. If the attorney for the Commonwealth consents to the reading of the affidavit on the hearing or trial as the deposition of the absent witness, the hearing or trial shall not be postponed on account of the witness’s absence. If the Commonwealth does not consent to the reading of the affidavit, the granting of a continuance is in the sound discretion of the trial judge.