If after you get the airship you go to the temples again and speak to all the fayth a second time (with the exception of Bevelle and Baaj Temple) the fayth will talk to you again and tell you interesting things about the story of the game.

If you speak to (I think) the fayth in Kilika, he will tell you that Jecht was a dream of the fayth. And that when Jecht touched Sin he became real, and thus was no longer a part of the dream. The fayth will also tell you that Sin could somehow go to the dream Zanarkand, perhaps to calm it.

And, if you speak to Auron on the airship several times right before you go to fight Yu Yevon, Auron will tell you the reason behind Jecht bring Tidus into reality. Something about giving Tidus life, hard as the journey was, it was still life.

And I agree with Ultima Seraph about the effects of the toxin.