From the preview by Xbox World 360:
"Although the game's a giant sandbox it's not all open from the start. The second half of the map is unlocked after twelve missions, and will be much darker than the first half. Once open, you'll be able to hop between the two worlds at will. The malaria hallucinations and combat 'freak out' sections both hint towards the protagonists' decent into madness, and having seen some concept artwork we're quietly confident that the latter part of the game is going to be genuinely creepy and full of scares, without resorting to the old 'trapped in a dark cellar with a mad zombie' ruse."
"We've admittedly only got down and dirty with the PC version so far, but after questioning the producer about the 360 conversion he said something quite magical: 'It's the same game. It will be the same.' Superb."