brought together some of the show's brightest lights and most celebrated stars for Monday's «Great Reunion» special, celebrating the show's 20-year history. ET was at the star-studded event, and spoke with some of the alums about their most memorable moments from their journeys.Justin Guarini — who finished as the runner-up behind Kelly Clarkson during the show's first season -- spoke with ET's Matt Cohen about returning to thestage, and how it felt.«Being back on the stage — that looks nothing like the stage that I was on by the way, they put a little budget into it this year and the years since I've been on it -- it's amazing to be back and see that the energy hasn't changed,» Guarini shared.«My favorite memory from my time on has to be my very first performance that I did live on television, because we didn't really know if [the show] was going to be anything,» he recalled.