Grant Gustin played the devilish Warbler Sebastian last season on 'Glee,' setting his sights on Darren Criss' Blaine. He served as the guest mentor on tonight's (July 17) episode of 'The Glee Project,' which centered on theatricality, telling the hopefuls that the key to the concept is to sell it without overselling it.

Gustin understands the notion and concept of theatricality, since he was plucked from a production of 'West Side Story' and dropped into 'Glee.' Here's why he was the mentor for this episode and theme: He famously referred to the happily coupled up Blaine as "sex on a stick" and tried to woo him from Chris Colfer's Kurt. Despite his not-so-good-intentions, Sebastian was the character you loved to hate, as opposed to flat out hating, thanks in no small part to Gustin's abilities. He was able to sell it without overselling it.

When Gustin walked into the room to explain the homework assignment to the remaining eight contestants, they were over the moon to see him. He was totally flattered by their reaction, since he was a supporting castmate and a bit player, relatively speaking, on the series that they are trying to get on.

The singer/actor/Broadway vet told the remaining hopefuls that the essence of theatricality is using music and dance to express yourself. "It's the height of emotion, and song and dance is all you can do to express it in the moment," Gustin said.

The hopefuls performed 'I Hope I Get It' from 'A Chorus Line' and he ultimately chose the adorable Ali as the winner. It was her first homework assignment victory. During their one-on-one mentoring sesh, he told her, "Don't let the style of music stop you from emoting during the song."  Gustin's final piece of advice? Always bring honesty into the performance.