Michael Slovis says flooding forced him to rethink a major scene at the last minute: "Our little trickle of a stream was now a raging torrent of a river."

[Warning: Spoilers ahead for Sunday's episode of Game of Thrones, "The House of Black and White."]

Game of Thrones is wasting no time shaking things up this season.

It's been a long time since an episode featured major developments for so many characters as Sunday's did. Among the big moves:

Jon (Kit Harington) was elected Lord Commander of the Night's Watch; Daenerys (Emilia Clarke) nearly started a riot after publicly executing a young murderer; Jaime (Nikolaj Coster-Waldau) embarked on a covert mission to rescue his daughter Myrcella (Nell Tiger Free) from Dorne; Brienne (Gwendoline Christie) found Sansa (Sophie Turner) and learns the young Stark wants nothing to do with her; and the biggest fan service moment of them all: Jaqen H'ghar (Tom Wlaschiha), not seen since season two, returned to begin training Arya (Maisie Williams) in the ways of the Faceless Men.

In a chat with The Hollywood Reporter, director Michael Slovis talks about crafting those big moments, why the fight scene with Brienne turned out completely different than he planned, and the challenge of introducing Dorne, the faraway homeland of season four fan favorite Oberyn (Pedro Pascal).

There's a lot of tension when Brienne meets Sansa. How did you convey what all of those character's wanted?

You have so much going on in that scene. Everybody had an agenda in that. You had to make it clear what each person's cause celebre was. Nobody is really saying what they mean in that scene. One of the great things they do on Game of Thrones that is not available on any other show I've been on, is [having] a rehearsal day. The art department marked out the studio in the dimensions of the inn, and we spent two hours rehearsing it as if we were doing a movie or a play.

You then immediately go into a horse chase. What were some of the challenges of that?

We had actors who could and could not ride. Or were not allowed to ride because it's dangerous to ride down those hills that quickly. I had this incredibly beautiful image in my head that the entire fight was going to happen out in the middle of that stream, where at the end Poderick gets to the water and his horse won't cross.

When we first went to the farm where we shot that, it was a teeny, tiny trickle of stream. We built a dam further down the river, and we cut out with a backhoe a space for the guy — after he gets stabbed in the neck — to fall into so it would be safe. And we did all of this planning, because they were going to be sloshing around in the water for this fight.

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Judging from the episode, that's not how it turned out.

On the way into work that morning, the stunt coordinator Rowley [Irlam] called me in the car and said, "We can't shoot the fight the way we planned it." Our little trickle of a stream was now a raging torrent of a river, because of the rains of the previous weeks. Even a modest sized show has trouble changing direction in short notice. But certainly something the size and complexity of Game of Thrones has an even harder time. To everybody's credit, we shot our horse chase that day, and we went in and we reblocked the fight.

Did you always know Sam's election speech would be a total scene-stealer?

I did not know ahead of time how he was going to do it and pull it off, but I had a version in my head that was not as quietly impassioned as John [Bradley] was. When we shot it, I just was blown completely away, and I knew that it was going to be the centerpiece. That is the moment when that scene completely turns. He single-handedly guides it and turns it.

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Sam has proven to be much more heroic than he thinks.

Especially coming off of that lovely moment in the end of episode one when Mance [Ciaran Hinds] is burning and Gilly [Hannah Murray] turns her head into his shoulder and looks to him for comfort. And you follow through with the scene in the library, which was also a very interesting scene, where the whole idea of leadership is a central theme, and he's talking about the youngest boy to be elected Lord Commander. I didn't even realize when we were doing it that it all summed up into that little speech Sam gave for Jon.

The confrontation in Dorne between Doran (Alexander Siddig) and Ellaria (Indira Varma) seems likely to have big implications for future episodes. How did you get that tone right?

All guest directors depend very heavily on showrunners to fill them in — to tone us in an appropriate way to do this. They guide me in what we call tone meetings about what they're trying to set up for future episodes, which is equally important to what's happening in my episode. I can say, "He should get pissed off and stand up and rant and rave," and they can go, "You know what, we've thought of that. But four episodes down the road, this happens and we're [saving] that kind of emotion for then." I'm just using this as an example; this is in no way tied in specifically to the story. But my point is that they have the really big picture so they can get me and my episodes to fill in the space between the last episode of season four and to be an interstitial.

Stay tuned to the Live Feed for more from Game of Thrones, and read our Q&A with Tom Wlaschiha, who discusses his return as Jaqen H'ghar.

Game of Thrones airs Sundays at 9 p.m. on HBO.