John Calvin Commentary Acts 25:12

"Then Festus, when he had conferred with the council, answered, Thou hast appealed unto Caesar: unto Caesar shalt thou go." — Acts 25:12 (ASV)

Festus having talked with the council — the governors used to have certain of the chief citizens who attended them and sat with them in judgment, so that they might decree nothing without the consent of the council.

Furthermore, it seems that Festus pronounced this with indignation when he asked, Hast thou appealed to Caesar? This was, namely, because it grieved him that he could not do the Jews the favor he desired. However, I leave that point undecided, because it is not of great importance and leans only on conjecture.