During Tyrion's trial recess, Jaime makes a deal with their father that Jaime will leave the Kingsguard and resume his place at Casterly Rock as the family's male heir if Tyrion stays alive. Their father agrees, saying that after Tyrion's trial, he will be sent to the Night's Watch.
Jaime shares the plan with Tyrion who remarks, "Ned Stark was promised the same thing, and we both know how that turned out." Jaime has no words and simply asks Tyrion to trust him.
The trial continues with Tyrion aligned with the deal, until Shae, Tyrion's mistress-turned-spy for their father, testifies to a story so egregious that Tyrion is unable to stomach the lies. He lashes out at everyone in attendance, sarcastically admitting to the murder, and demands a trial by combat — putting Jaime in difficult position.