“questioned him at considerable length.” The Greek reads, “was questioning him with many words,” but the phrase means “questioned him at considerable length,” (NET; cp. “great length,” CJB; “some length,” ESV, NASB, NRSV). Herod questioned Jesus for a considerable length of time, but there is no evidence of a formal trial. Herod wanted to have his curiosity satisfied, and did not really care whether or not Jesus got justice in his court.
“but he did not answer him.” It is certainly understandable that Jesus did not answer Herod Antipas. First and foremost, there was no profit in it. For him to die in Jerusalem he would have to be tried before Pilate, who had the authority there. He had no desire to be taken under arrest back to Galilee. Furthermore, there was no point in providing any satisfaction to Herod, who had killed his friend and cousin John the Baptist. Neither was he interested in giving any satisfaction to Herod’s court, which included his murderous wife Herodias, or his dancing step-daughter Salome. He could have told them that soon he would be their judge and condemn them to a second death, but that would have only made him the butt of their jokes.