As soon as the camera rolls on the first take they have to solidify what they’re doing for the whole scene. Technically, it is one of the most intense and difficult acting jobs I’ve ever witnessed, on just a technical level. That doesn’t even include the stellar acting that Tatiana has to pull out in the scenes — and that she does, successfully, every time — because the scenes can’t be longer or shorter when you run them. They have to run at the same pace for editing and for placement of the multiple Tats. She has to move her hand at the right moment to grab an object, it has to be at the right height, it can’t be an inch too low, and it has to be the same every single time. And this could be like two minutes into the scene, so she has to pace the scene perfectly so that at that two minute mark her hand is gonna be right here to grab the bottle or do whatever interaction we love to do in those scenes.
So it is incredibly difficult, it is timed to the second, all the body movement has to be exactly right. She does the scene with her body double, then we take everybody else out — we have earpieces — she gets the lines from that earlier scene and she has to act with a tennis ball on a stick for eyeline. Because the techno-dolly is capturing all of this in the exact same way every time, we have these plate shots and she can watch on the live monitors the old shot of her and then, as she does the switch-over, we’ll see the new shot with her new body and she’ll run around the camera, take a look at the last take and be like, “Oh, my hand is too high and it comes too early. Ok, I need to remember when I get to that moment to have it lower and blah blah…” runs back, action, NAILS IT. She has to get every single moment. It’s absolutely insane. I don’t know how she does it.
Kristian Bruun, offering the best description I’ve heard of the multi-clone filming process (via sestra-marie)