The most important thing that needs to be executed correctly from the novel to screen adaptation of Ender's Game is the Zero G battle room. But how on Earth did director Gavin Hood shoot this? Ender himself, Asa Butterfield, explained everything in our exclusive interview.

We sat down with Butterfield and talked mind games, freeze rays and battle scenes, at CinemaCon. Filming Ender's Game sounds like it was exhausting.

How did you film the battle room scenes?

Asa Butterfield: They're really fun to film. Everyone on the cast was excited to start when we first started. The first few weeks weren't in the battle room, we filmed the battle room scenes in the middle. We did it in order of when the sets were finished building. We had training in rehearsal, it was in harness. Harnesses aren't the most comfortable things you can wear, and neither are the flash suits. They look incredible, when you're wearing them. [But] you're very hot, to say the least. It was hard work, we had a lot of physical training. And we spent a lot of time practicing getting the movements and pretending to be in zero gravity.

An astronaut came in once to talk to us about moving in zero gravity and what it's really like in space so we could give the most believable performance. You have to move really slowly…fluidly and smoothly. When you're in the harnesses to stop yourself from falling at the waist, which is where they're connected, you have to be tensed up. So keeping actions smooth whilst having your whole body completely tensed is surprisingly difficult. Meanwhile you're saying your lines. They were fun, they were amazing fun to shoot, especially the sort action moments when you're flying across shooting your gun. Those are awesome.

Speaking of the [Battle Room] gun, I know in the book when you shoot it, it freezes a part of the body [if it hits the target]. How did you act that out? Are they going to CG your body so it's a different color [when shot]?

For the long shots you just had to freeze. You're just frozen. For the close ups occasionally they had these contraptions that just locked your arm in place. And they did use CG to make sure there weren't any tiny wobbles.

That has to be hard frozen, to be in the air, holding that shot.

You just get used to it, you just hang in there. It's a bit easier than having to move more fluidly. You just freeze, and once you've tensed, you just stay like that. So you're just flowing there. If you start spinning then you have to make sure you're constantly spinning with the same momentum.

In the novel Ender's Game takes place over a couple of years, how did you handle that? Did they hire actors to play younger versions of you or older versions of you?

From the book, there were things that had to change. The most obvious thing was that they had to age the characters up. Because you can't find — at least as not that I'm aware — a six-year-old actor that can play that sort of a character, it's complex and it's physically demanding. That was the first issue, so they changed it from being six to 13 or 14. Then again, the time frame in which the story takes place had to be shortened. Because there are so many key actors for the kids, you would have to get five or six different actors for the ten different characters. They just shortened it to the space of about a year rather than the 6 or 7 years that takes place in the novel.

Harrison Ford has a very big role in this film [as Colonel Hyrum Graff.] Was there any sort of hazing he put the kids through?

Harrison is an amazing guy, he's really sweet and he's an incredible actor as well. Both him and Sir Ben [Kinsgley] were really inspirational to work with. They were both really professional and they stayed in character, which really helped us to give our best performance.

That sounds intimidating…

It was — and then again, it was helpful. It helps you give the right performance, rather then you acting like best friends, which is what it felt like when we weren't in character. We did get close. But you stayed in that sort of slightly distant relationship. And there's a conflict between Ender and Graff that we kept, in-between takes.

What was the hardest form of psychological torture for Ender?

Obviously there's the ending, which I'm not going to say. But that's definitely the biggest. But [out] of [the] things I can talk about, probably one of the hardest scenes for Ender was leaving his sister, back on Earth. I think that's one of the things that really damaged him. As you can see in the mind game, as well. Which is a really interesting aspect of the film and the story as a whole. Ender's relationship with Valentine ends up slightly rep