EXCLUSIVE: 'Shadowhunters' Star Katherine McNamara Dishes on Sarah Hyland Joining the Cast!

McNamara opened up to ET about the exciting casting news: 'We've all been so excited about it!'

Sarah Hyland
is signing on to Shadowhunters, and star Katherine McNamara is opening up about the Modern Family actress' involvement with the sci-fi fantasy drama.

McNamara sat with ETonline's Leanne Aguilera for a Facebook Live chat on Monday, and played super coy about which of her co-stars is a better kisser, Dominic Sherwood or Alberto Rosende.

"I can't answer this question, I can't do it. I have to plead the fifth on this," McNamara said, laughing. "They're both amazing though, I can tell you that!"

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However, one reason she might have been hesitant to answer the question could be the fact that Sherwood and Hyland have been dating for two years and there's no reason to upset your show's newest castmember.

Hyland announced the news on Monday in a video she posted to Twitter. "Yup, you heard it Shadowhunters fans, I'm gonna be on the show!" the 26-year-old actress exclaimed. "I can't say who I'm playing yet because it's a secret."

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McNamara was equally evasive regarding details of Hyland's character, but she did explain how much of a relief it is to be able to talk about her joining the cast.

"We've been keeping that so secret for so long. It's been so hard," she explained. "I'm really proud of us because we've all been so excited about it and it's been really hard to keep it secret, but she's been a part of this family since day one. And so having her on this show was only natural."

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However, McNamara couldn't dish on when Hyland would be debuting on the show or anything about her character.

"I'm sorry. I want to tell you, I really do! But they'll kill me," the 21-year-old actress said. "You guys are going to be so happy. I cannot wait for you to see this."

airs Mondays at 8 p.m. ET/PT on Freeform.

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