With Sean Hayes stepping into “Smash” this season, who is not expecting—hoping for—a “Will and Grace” reunion? After all, Debra Messing was funniest when she was goofing off with the flamboyantly lovable and annoying Jack McFarland. The already musically-talented Hayes, who was in the 2010 revival of “Promises, Promises,” is joining the cast as “Terrence Falls,” a comedic television and film star who is about to make his Broadway debut in the new musical, “Liaisons.” Rumors state that Hayes’ character is going to be a nuisance for Ivy Lynn (Megan Hilty). Wouldn’t that make for a great cat fight?