Some spoilers may follow, just so you know.
Many cutscenes either fade to black or white, to subtlety let us know if what we are seeing is real or not. After going back and reviewing all the cut scenes (if you planing on doing it yourself then do what I did and steal a page from "A Scanner Darkly" and play each chapter in reverse order, start at the epilode and work your way back to to Chapter one, less likely to get in a groove and forget one) I noticed something. Chapters 2 through 6 do not use the white fade at all, but Chapter 1 has several.
The first cutscene after gaining control of Walker ends in white, the mayday call from the 33rd also has multiple white fades, and the outro cutscene for chapter 1 opens white but ends black. So what is happening here? It can't completely be a hallucination as the Mayday is brought to Walkers attention by Lugo and both teammates show no sign of confusion or that anything is amiss. What is really happened? My theory is that Delta heard a comunication that hinted that the 33rd were in trouble but it wasn't a mayday and Delta never replied to it. Walker saw trouble and wanting to be a hero desided to help, he just imagined responding to a mayday as justification.
There arew a few other areas where a white fade is used but there isn't anything to hint that's not real, like when you meat Radioman, it might be the white isn't used exclusively for hallucinations. Anyone else got any ideas on what going on in these cases?