"The entire marketing campaign for Metal Gear Solid V has been about changing or hiding an identity, and so its clear - you don't play as Big Boss in The Phantom Pain.
Even the announcement of MGS V was smoke and mirrors. It was announced as Ground Zeroes at the Penny Arcade Expo way back in 2012. It was clear that it would be a prologue to Metal Gear Solid V, but it was never shown to be less than an hour long or set in a single location. Konami and Kojima spoke about shorter cutscenes, an open world, travelling between countries and the worry that Ground Zeroes might just be too violent for release. "