Jaime tinha um pressentimento de que Cersei não iria realmente matá-lo

Nosso bom companheiro Nikolaj Coster-Waldau (o ator que interpreta Jaime) explica isso bem para nós uma recente entrevista com Leigh Blickley em HuffingtonPost :

[Leigh] Not going to lie, I thought this might have been your demise. The Mountain was ready to kill you! But you called her bluff.

[Nikolaj] Cersei had the chance with both her brothers to kill them and she was threatening to do so, but both times she eased off on the trigger. I get that with Tyrion she was setting him up for failure ― making him believe that he was the hero of the day, that he negotiated this amazing support from the Lannister army and what a wonderful thing he did. So that makes sense. For Jaime, I would think he would think there was part of her that still understands that he really is the last person that she has. That is the truth; that must resonate somewhere inside her. That would help. Or maybe the Mountain is just so thick that he didn’t get the eye signal. Maybe there’s a scene right after where she goes, “Why the fuck didn’t you kill him?! I gave you the signal!” I don’t know. Who knows. We’ll never know because that scene isn’t there, but that might be the case.
_{-HuffingtonPost, Nikolaj Coster-Waldau Breaks Down That Huge Jaime Moment On 'Game Of Thrones'.}

Não sei se a citação dos scripts vazados é uma boa ideia. Mas, como o episódio já foi ao ar, aqui está a linha do script vazado :

[Jaime and Cersei]: "He walks out on her, while she is threatening to have The Mountain kill him. Jaime calls her bluff however, and the Mountain stands aside and lets him leave her chambers. Next we see him he's on his horse heading north. That's all we see of him."

Então, Cersei estava blefando, e provavelmente não era o seu "Go Ahead", e apenas Sor Clegane sabia.

Você pode ver Ser Gregor recebendo instruções no início do episódio sobre o que fazer se algo de ruim acontecer "Mate a puta de cabelos prateados primeiro e depois ... etc". Então o grande cavaleiro-zumbi pode seguir instruções básicas. "Se eu acenar para você - tire sua espada, mas não o mate" seria igualmente fácil.

Porque Cersei nunca deu a ordem

A ordem que vemos na tela é Cersei dizendo à Montanha para sacar sua espada. A ordem de matar nunca vem. Isso é confirmado no script oficial .

JAIME: Give the order, then.

He watches her and waits.

The Mountain waits for a command from his queen.

She nods. The smallest possible nod.

The Mountain draws his sword.

Jaime knows there’s no point fighting. He knows he will be cut in half.

He stares at his sister for a long beat.

JAIME: (quiet) I don’t believe you.

After a long beat, Jaime turns and walks away, right past the Mountain and his drawn sword.

Cersei watches. She does not give the command.

_{Game of Thrones, Season 7 Episode 7, "The Dragon and the Wolf"}