A few episodes ago, Red Witch said Stannis would need to sacrifice his daughter for the King's Blood. He asked if there was any other way, such as leeches. She said it wouldn't be enough this time.
I get it if you want to debate whether or not Stannis would actually go through with it, but I don't understand any confusion about where this actually came from. A lot of things get implied on this show...this was explicitly stated.
I don't think I expressed any confusion about where it came from. I expressed frustration about what I thought was a shoddy job by the storytellers in getting the characters and the circumstances to that place. The "explanation" you cite is a good example of that. It's not a real explanation. It's a contrived line with no basis in anything else we've been told.
Why isn't it enough? We're not told. Why does the method of sacrifice jump from obtaining kings' blood (the reason Shireen was chosen for sacrifice) to one of the few methods of killing someone that makes it virtually impossible to obtain a decent quantity of their blood? Again, we're not told. Why did it have to be done in public ceremony in front of a mother who will obviously be devastated and troops who will probably find it a bit disheartening? Again, we're not told.
I'm fine with doing all those things, but I think you have to explain why you're doing them. Even if the explanation is as simple as "the Lord of Light demands fire instead of blood this time, and he demands that the sacrifice and her relatives suffer as much as possible. And hey, your troops can roast marshmallows on your daughter's smoldering flesh, that will be good for morale!" At least say it on screen. Don't hide it. That's a cheap trick to heighten the emotional impact of scene IMO.