In Hideo Kojima's most recent Famitsu interview about his upcoming game, Metal Gear Solid V: The Phantom Pain, he discusses the main heroine of the game, Quiet. When the interviewer asked Kojima-san about her importance in Metal Gear Solid V and about her motion capture actress Stephanie Joosten he answered:

In the game, Quiet is the main heroine. Whether she is friend or foe has yet to be revealed so I had to be careful when casting her role. MGS normally has old timers and old guys (laughs). Stefanie is very skilled in action but since she never held that type of gun, we asked her do some training at home.

Kojima-san also provided a few more details in a separate interview on 4gamer:

With Stefanie, we had to make sure to focus her 3D motion on capturing her facial and body language and not her voice, since Quiet does not speak. She does make small sounds like "ugh" and "ah" though.

I can imagine that some people will cry foul over the idea of a main female protagonist having no real speaking lines. The popular archetype of "strong and silent" has, without a doubt, been prominently a male role, which is most likely because writers fear that while a man can still have a strong presence without speaking, a woman playing that same role would be seen as "sexist."

I believe, however, that this assertion is actually the sexist one. Women are not required to constantly speak and the idea that a woman has to talk a lot to be a proper female is quite offensive. It's refreshing to see a strong female character that is both capable on the field and capable of expressing herself and communicating with others without a single line of dialogue.