The mission Edge played saw the protagonist, Jason Brody, hallucinating while trying to take out enemies. He was still able to stab two guards then throw the knife at another -- something we doubt we could do while (almost) perfectly lucid.
The main antagonist, Vaas, is "a sociopath, and very nearly a psychopath," Edge says, and the Ubisoft development team wants the game to be "emotional and raw" while still remaining a shooter first. There will be no weapon degredation, Gaming Everything adds. Check out the entire run-down of Jason Brody's descent into mushroom madness right here.