Ryan Kiera Armstrong is set to star in the 'Buffy the Vampire Slayer' reboot

Sarah, 48, wrote on Instagram: "I want to introduce you to @ryankarmstrong From the moment I saw Ryan's audition, I knew there was only one girl that I wanted by my side. To have that kind of emotional intelligence, and talent, at such a young age is truly a gift. The bonus is that her smile lights up even the darkest room. Welcome to #NewSunnydale #btvs (sic)"

Ryan will play the new slayer in the Chloe Zhao-directed pilot. The actress' character has been described as an introverted high-school student.

Sarah actually broke the casting news to Ryan, and she's posted a video of their conversation on Instagram.

The actress said to her new co-star: "How do you feel about helping me to save the world? You want to be my chosen one? Will you stand by my side and save the world? Yeah?"

Sarah revealed that Ryan's talent shone through in her audition, and she's now looking forward to teaming up with the young actress.

She said: "Ryan, we all love you and from the second we saw your tape, there was nobody else in our eyes that could do it, that we wanted to do it with. And I'm so excited to stand by your side and have you be my partner on this journey ... So I hope it's a yes!?"

Ryan - who broke down in tears after receiving the news - then replied: "Yes! Yes! Yes, for sure. Thank you - I'm so excited!"

Nora and Lilla Zuckerman, the writers and ex ecutive producers of the 'Buffy' reboot, can't wait to work with Ryan on the show, describing the teenage star as the "chosen one".

They said in a statement: "We are so overjoyed to have found this generation's slayer in Ryan Kiera Armstrong. She absolutely blew us away - there is no question in our mind that she is the chosen one."