Sebastian, who is usually a passive character, beats up Sir Toby and Sir Andrew because, mistaking him for Cesario, they attack him first (Act IV, Scene i, lines 24-43).
Antonio, who is a devoted friend to Sebastian, risks his own life when he follows Sebastian to Count Orsino's household in order to protect him. Antonio has "many enemies in Orsino's court" (Act II, Scene i, lines 45-48), and often has to hide so as not to be recognized. Later in the play, Antonio enters the scene just as Cesario and Sir Toby are commencing a duel. Thinking that Cesario is Sebastian, Antonio tries to help him. Officers come and recognize Antonio, and he is arrested (Act III, Scene iv, lines 327-332).