Glee: Darren Criss and Naya Rivera on "Moving" Cory Monteith Tribute and Demi Lovato's Guest Role

Exclusive! Rivera reveals that filming the episode has been "the worst week ever"

As we reported last week, Glee is well into their filming of the episode that will serve as a tribute to Cory Monteith, who died tragically over the summer. And based on what the cast is telling us, just getting through filming will be a triumph in itself.

"It's the worst thing that's ever happened to me," Naya Rivera admitted during last night's MTV Video Music Awards. "It is the hardest thing to do. It's been the worst week ever."

"We're shooting it right now. It's tough to shoot," Darren Criss adds. "It's something that needs to happen for the fans. I don't know; it's going to be tough. But it will be very poignant and hopefully very moving, and just that: a tribute. Hopefully it will be a good tribute to Cory."

Sources told us that the episode, which is reportedly titled "The Quarterback," will have viewers bawling "from beginning to end," so we can't even imagine what it's been like for the cast to film the songs and scenes dedicated to Finn Hudson. Lea Michele, who we're told appears in the episode in a "limited but incredibly powerful" way, recently tweeted a photo of her character Rachel wearing a Finn necklace.

Besides the Cory tribute, Glee is busy adding great guest stars to their fifth season, most recently X Factor judge Demi Lovato. Her addition was a surprise to both fans…and the cast!

"I found out on Twitter! I guess that's just how things break," Rivera laughed to us. "I'm excited. I'm so excited to work with her. We have a duet together, and we're going to be working very closely, so it'll be awesome."

Demi's character and Santana are rumored to be singing "Here Comes the Sun" during one of The Beatles episodes. Lovato tweeted about joining the show today, saying she was "wayyyyy too excited" to have a guest role in season five.

"I can't believe I'm going back to acting but the best part about it is that I get to act/sing with some of my really great friends.."

To hear more from Naya and Darren, and to find out how Adam Lambert feels about possibly working with Demi, watch our exclusive interviews below