As far as I can see, Zack appears only in a flashback(thankfully he just appears, being a large contrabution to the games story line). He is basicly a memory and a flash in the back of Clouds mind to remind the blond just how far he has come, and what people lost for him to get this far- atleast, I believe that is the role that the flashback will have. I know Zack appears atleast twice, both in flashbacks. One is the scean in the back of the truck, as you can see on the trailer, where he asks Cloud "We're friends, right?". The other [spoiler]he is running through a desert like area, his sword raised, and shouting at Cloud to run.[/spoilers]. This information i read off from a site that was spoiler ridden, so the moment I can find the link again I will post it in this topic. As far as I know, Zack will only appear in flashback- and a good thing too. He is very much dead. And like I previously stated- Zack is probably only being shown to help Cloud see that he would have wasted lives if he just simply gave up.