FAQ Evento Stark
This event can only cancel an ability with the word "choose" in its text (so it won't work with Tears of Lys, Ghaston Grey, etc.).
The targeted Stark character doesn't have to be kneeling (standing is not a cost here, unlike with Ser Barristan Selmy).
You can use this event to cancel your own ability (from an event or a card in play) that targets a Stark character you control, just to stand that character.
You cannot use this card to cancel the ability of Ser Gregor Clegane or The Tickler (because of the word "Then": when the ability initiates, it doesn't target anything yet, and when it does target, it has already initiated some time before that).
You can use this card to cancel Intimidate or Stealth (keywords are abilities).
De lo que entiendo que si lo cancelaría porque la trama dice "choose".
This plot's Interrupt ability is triggered after the winner of the challenge is determined, but before he is "declared". Killing the only defender will not make the challenge unopposed, or let you win by 5+ STR -- these factors have already been fixed.
The interrupt is triggered before any "If you win the challenge..." delayed effects resolve (e.g. The Lord of the Crossing), and before any "After you win/lose a challenge" Reactions can be triggered.
If you trigger "The Rains of Castamere" and reveal The Red Wedding, it is too late to use the plot's Interrupt ability.
Importante tener en cuenta el ultimo punto de las FAQ de la trama para aquellos que juegan RAINS.