nobodysuspectsthebutterfly:

I have, it’s a favorite interview. I’ve also answered this question before, but not for a long time, so you get it again, this time with extra quotes and sources.

So, first and foremost – Sansa is not to blame for her father’s death. The only ones to blame are Joffrey, who gave the order; Ilyn Payne, who obeyed without question; and almost certainly the man who convinced Joffrey to go against the plans of Cersei and Varys and the small council to allow Ned to confess treason and take the black, the man who made sure Ilyn Payne and Janos Slynt were ready and waiting for the execution.

However, Sansa did unfortunately have a part in the events that led up to Ned’s execution. As did many others, not in the least Ned himself – because, after all, he did tell Cersei he knew of the true parentage of her children, and told her he would be telling Robert as soon as he returned from his hunt, and told her to flee with her children before Robert killed them all. Ned also made plans to send his daughters to safety, preparing a ship for them to travel back to Winterfell (because, unlike the common paradigm, Ned was not stupid, just… outwitted, and too merciful). However, when Robert returned several days later, deathly wounded, Ned did not tell him (he didn’t want to break his heart in his last moments on earth), only modified his will to refer to Stannis and not Joffrey. The next morning (an hour before he was informed Robert had died), Ned told Arya and Sansa that he was sending them away from King’s Landing by noon. He allowed Arya to have one last lesson with Syrio (who was coming with them), but told Sansa she could not see Joffrey before they left. And Sansa fled, crying, and went to Cersei:

She was the good girl, the obedient girl, but she had felt as wicked as Arya that morning, sneaking away from Septa Mordane, defying her lord father. She had never done anything so willful before, and she would never have done it then if she hadn’t loved Joffrey as much as she did. […] The king had been her last hope. The king could command Father to let her stay in King’s Landing and marry Prince Joffrey, Sansa knew he could, but the king had always frightened her. He was loud and rough-voiced and drunk as often as not, and he would probably have just sent her back to Lord Eddard, if they even let her see him. So she went to the queen instead, and poured out her heart, and Cersei had listened and thanked her sweetly… only then Ser Arys had escorted her to the high room in Maegor’s Holdfast and posted guards, and a few hours later, the fighting had begun outside. –AGOT, Sansa IV

She was not privy to all of Ned’s plans regarding Stannis, the gold
cloaks, etc… but she knew more than just that her father planned to
spirit her and Arya away from King’s Landing. She knew when they were to
leave, on what ship, how many men would be in their escort, who would
have the command, where Arya was that morning, etc… all of which was
useful to Cersei in planning and timing her move.

–GRRM

Because Ned did not tell his daughters why he needed them to leave King’s Landing, because he did not tell Sansa that Cersei was an enemy, because Sansa trusted the queen and was desperate to stay with Joffrey, Cersei was able to draw out all of the information she knew. You can imagine the conversation: “Oh, you’re leaving, my dear?” “Yes, Father is making me go, you have to make him change his mind, I love Joffrey and–” “Yes, of course, sweetling, but tell me, how has your father said you will be traveling?” “Oh… there’s a Myrish ship, the Wind Witch, it’s leaving today–” “Surely your father isn’t sending you on a ship with foreign sailors, all alone.” “No, Septa Mordane and Jeyne and Arya are going too, and there’s to be a troop of ten men for our protection, led by–” “And where is your sister now, dear Sansa?” “Arya’s having a lesson with her dancing master in the Small Hall, even though she gets to bring him with us! It’s not fair…”

And after that interrogation disguised as concern, Cersei took Sansa prisoner. At the very moment Ned was making his stand in the throne room, Sansa was under armed guard in Maegor’s Holdfast. Even if, somehow, Ned had been able to escape Cersei’s coup, Janos Slynt’s betrayal, and Littlefinger’s dagger, somehow managed to get out of the Red Keep with a few men, Sansa would have already been a Lannister hostage.

As it was, because of Sansa’s information, Cersei’s men killed Syrio Forel, and killed the Stark men at the Wind Witch and set up a trap for Arya. As it was, after Ned was taken prisoner, he was quite prepared to die. He was not going to capitulate and betray his word, to tell the world the lie that he was a traitor, that he’d lied about Joffrey’s claim to the throne… until Varys reminded him about Sansa:

“So what is your answer, Lord Eddard? Give me your word that you’ll tell the queen what she wants to hear when she comes calling.”
“If I did, my word would be as hollow as an empty suit of armor. My life is not so precious to me as that.”
“Pity.” The eunuch stood. “And your daughter’s life, my lord? How precious is that?”
A chill pierced Ned’s heart. “My daughter…”
“Surely you did not think I’d forgotten about your sweet innocent, my lord? The queen most certainly has not. […] Lord Eddard, tell me… why is it always the innocents who suffer most, when you high lords play your game of thrones? Ponder it, if you would, while you wait upon the queen. And spare a thought for this as well: The next visitor who calls on you could bring you bread and cheese and the milk of the poppy for your pain… or he could bring you Sansa’s head. The choice, my dear lord Hand, is entirely yours.”

–AGOT, Eddard XV

So, to save his daughter, one of his children who he loved more than life itself, Ned came to Baelor’s Sept, to confess his “treason”, take the black, and be sent to the Wall. And Sansa was there too, proud that because of her desperate plea for her father’s life, because of the love of her dearest Joffrey, because of the kindness of Queen Cersei, because she’d been a good girl and done as she’d been told and written Cersei’s letters to her mother and Robb to ask them to stop the fighting, that her father would be safe, and everything would be all right. Until the moment Joffrey smiled at her, and called for Ilyn Payne.

Afterwards, alone in the room in Maegor’s Holdfast, Sansa became suicidal with grief and guilt, knowing how her trust had been betrayed. Afterwards, Joffrey told her that Cersei had said she was stupid, and Sansa realized that the queen’s kindnesses and promises to help had only been lies. Afterwards, Sansa understood:

Once she had loved Prince Joffrey with all her heart, and admired and trusted his mother, the queen. They had repaid that love and trust with her father’s head. Sansa would never make that mistake again. –ACOK, Sansa I