At the end of Final Fantasy VII: Advent Children when Cloud is in the healing water in the church with the kids getting their Geostigma cured, he sees Aerith and as she was away we see that Zack is leaning against the doorway (can be seen in this video on Aerith).
When i first saw the film i don't recall Zack having any lines but there is Final Fantasy VII: Advent Children Complete with extra scenes which i have yet to see yet (i've been told that Reno mentions meeting up with the other Turks like Cissnei)
So maybe i missed it in the original movie or maybe there's something only in the Complete version, but does Zack have any lines or even do anything at all besides show up at the end?