I think most of the flashbacks will just be referenced through dialogue. I think we saw a brief glimpse in one of the teasers of Brandon Stark strangling himself trying to save his father, and the only other flashback I can see them actually filming is the "Promise me, Ned" scene, and I'm picturing those flashes as being rather chaotic and jumbled because of Ned's fever.
I'll probably be completely wrong, though.
I remember reading that Aerys the Mad King was cast and the flashback of Ned's brother and father's death was filmed. The rest of the flashbacks will probably be told through talking ...
How much of Gregor's actions do you think they will actually show? Seeing a huge man bashing the skull of a little baby and then raping his Mother... Something tells me that isn't going to make it to air.
Yeah, I think it'll be too much even for HBO ...