Theophylact of Ohrid

Indeed, before the decisive war of the Romans against the Jews began, what did the apostles not suffer from them, being delivered up to courts and led before kings: Herod, Agrippa, and Nero? But you, He says, "do not worry beforehand what you will" answer: at that very hour the Holy Spirit will give you what you must say.

Commentary on Mark ↗

Gregory the Dialogist

But upon hearing so many terrors, the hearts of the weak could be disturbed, and therefore consolation is added when it is immediately subjoined: "Settle it therefore in your hearts not to premeditate how you will answer. For I will give you a mouth and wisdom, which all your adversaries will not be able to resist or contradict." As if He openly says to His weakening members: Do not be terrified, do not be afraid; you approach the contest, but I fight; you utter the words, but I am the one who speaks.

Forty Gospel Homilies, Homily 35 ↗

Bede

And when they lead you and hand you over, do not worry beforehand what you will speak, but say whatever is given to you in that hour. For it is not you who speak, but the Holy Spirit. When we are led to judges because of Christ, we must simply express our will for Christ. Moreover, it is Christ Himself who speaks in us, and the grace of the Holy Spirit is given in answering.

On the Gospel of Mark ↗

Glossa Ordinaria

Another anxiety might also arise in the breasts of the disciples. Lest therefore after hearing that they were to be brought before kings and rulers, they should fear that their want of science and eloquence should render them unable to answer, our Lord consoles them by saying, But when they shall lead you and deliver you up, take no thought beforehand what ye shall speak, but whatsoever shall be given you in that hour, that speak ye.

Catena Aurea by Aquinas ↗