This interview with executive producers David Benioff and Dan Weiss is a great read, as they discuss the upcoming “Blackwater” in some detail. There are some mild spoilers for those who haven’t read the novels, but a lot more of it is about the challenges of producing the battle. They’ve already previously noted that they received a “stipend” above and beyond the decided season budget to be able to put it together, but one interesting detail that I believe is new is the fact that originally “Blackwater” was due to be directed by another director until a personal emergency forced put an end to that. Neil Marshal was the name that popped up as a replacement, and it worked very well indeed.
I’m going to guess that the unnamed director was Jeremy Podeswa, who had at one point indicated (during, I believe, a red carpet interview) that Game of Thrones was on his schedule and then seemed to never be mentioned again in connection with the show. Podeswa’s credits include a number of HBO shows and miniseries, perhaps most saliently The Pacific in which he directed the episode “Iwo Jima”.
In any case, make sure to check out the interview, which includes another preview for the episode.