Later in the fourth season, when Tyrion stands trial for King Joffrey's murder, the brothers face another version of the moral dilemma that will haunt them throughout the series. Tyrion maintains the truth and his innocence even though he knows it will mean his life. Meanwhile, Jaime grapples with the opposing forces of duty and honour. Does he maintain his duty as the head of the Kingsguard or does he do the honourable thing of helping his brother?
It takes a nudge from Bronn for Jaime to gather the courage to visit his brother in the dungeons. Bronn tells Jaime that before Tyrion's trial by combat at the Eyrie for his crimes against the Starks, he'd initially named Jaime for his warrior until a twist of fate forced Bronn to take the mantle. According to the sellsword, Tyrion knew Jaime "would ride day and night to come fight for him," and Bronn encourages Jaime not to give up that fight when Tyrion needs him the most.
When Jaime finally visits Tyrion in his cell, he tries to comfort his little brother saying he had it much worse during his captivity. Jaime then questions his brother's innocence. Tyrion insists he played no part in King Joffrey's death. Then he laments that even if the real murderer confessed, Cersei would still want him dead.
The sentiment prompts Jaime to ask how he can help. Tyrion asks his brother to sneak him out of the city, but Jaime balks and insists that the truth will emerge at trial. They both know this isn't true, but each man remains bound by their position.