John Wesley on Acts 21:31

And as they went about to kill him - It was a rule among the Jews, that any uncircumcised person who came into the inner temple, might be stoned without farther process. And they seemed to think Paul, who brought such in thither, deserved no better treatment. Word came to the tribune - A cohort or detachment of soldiers, belonging to the Roman legion, which lodged in the adjacent castle of Antonia, were stationed on feast days near the temple, to prevent disorders. It is evident, Lysias himself was not present, when the tumult began. Probably he was the oldest Roman tribune (or colonel) then at Jerusalem. And as such he was the commanding officer of the legion quartered at the castle.