Game of Thrones Season 6 Discussion [SPOILERS WITHIN]

[SteelCurtain]

When?  Where?  I'm not saying you're not citing something true, I just don't remember this and am interested in looking it up.

Tyrion recalls a memory whilst he was at Winterfell, in which he overheard Joffrey Baratheon jesting with The Hound to send a dog to kill a wolf. He guesses that Joffrey hired the catspaw among those that joined the king's party as they made their way North and that he had found the dagger amongst his father's weapons and had simply taken it. However, Tyrion is unable to figure out why Joffrey wanted Bran dead, and attributes it to his innate cruelty.[2]

Jaime later realises that Joffrey sent the assassin in an attempt to impress his father, after overhearing a drunken Robert Baratheon say it would be kinder to put the crippled Bran Stark out of his misery.
http://awoiaf.westeros.org/index.php/Cat...7D.7D.7D-1

Tyrion puzzles out how his nephew must have found Littlefinger’s Valyrian dagger amongst Robert’s possessions, assuming that the king had probably forgotten he even owned it. Joffrey must then have paid some unsavory freerider to open Bran’s throat, though Tyrion still cannot figure out why, attributing it to Joffrey's innate cruelty for the moment.
http://awoiaf.westeros.org/index.php/A_S...Chapter_60

When Cersei mentions that Robert himself, at Winterfell, said it would be kinder to put the boy out of his misery while alone with her and their children, Jaime figures out that it was Joffrey, 'desperate for a pat on the head from that sot', who sent the assassin to kill Bran, even though Cersei does not believe him when he tells her his suspicions, just as Jaime does not believe her when Cersei insists again that Tyrion is responsible for Joffrey's death.
http://awoiaf.westeros.org/index.php/A_S...Chapter_72

From the books perspective, Jaime and Cersei have no reason to be lying to each other over this.