Ian Failes, for fxguide:

Ava is clearly intended to be a robot of some kind, but Whitehurst was adamant that she not feel robotic in terms of her CG materials. “The one rule I made from the outset,” he says, “was that no-one was allowed to look at robots. You were allowed, though, to look at things like Formula One suspension or high-end bicycles. We also looked at human anatomy, of course. Ultimately she’s a machine who is supposed to move and behave exactly as a human would. All of the muscles we have in there are simplified versions of human ones, for instance.”

“Initially the back of Ava’s head and neck were not metal,” adds Whitehurst, “but that decision was made to have the character weirder to look at. One of the topics or ideas in the film is that we wanted her to look robotic. When you are presented with that visually, do I read her as a character or do I read her as a machine?”

I’ve been digging the aesthetic of this film since I saw the first preview. The design of the character is elegant, and exotic without looking over-produced. The set design is also top-notch.