I think Zack was a ladies' man like Aeris said about him, and I think he was also quite talkative. Remember the flashback scene when "Cloud" kept talking to Sephiroth all the time when they were en-route to Nibelheim five years prior to the game? That was Zack. When Cloud and Zack were on their way to Midgar and Cloud was totally messed up because of the Jenova cells, Zack also kept talking a lot to Cloud and the driver of the truck.