Kristen Stewart, Mackenzie Foy and Robert Pattinson in a scene from the movie.
Foy, who turned 12 in November, had a difficult task ahead of her, according to the 10-minute video.
Although she was only supposed to be three-years-old, Renesmee was already a super genius. She could speak 20 languages and had read the works of philosophers like John Milton, according to the book.
Director Bill Condon explained that it would be impossible to find a real three-year-old who could handle the part, so they decided to take artistic license and cast Renesmee as a super-fast-growing child.
“When we were first casting Renesmee we were going a little older, but even then, how are you going to find someone who sounds like a little adult and convey the emotions we need,” explains author Stephenie Meyer.
Surprisingly, a lot of fans had suggested Foy for the part, according to casting director Debra Zane.
The video contains some of Foy’s heretofore unseen audition tapes and her own commentary about her thoughts on the part as well. “It was one of those things that as soon as we saw her that was it,” says Condon.
The video also provides a behind-the-scenes look at how a doll was used to play Renesmee as an infant and how they used computer generated graphics to help bring it to life.
Stewart infamously called them “little Chucky dolls,” during an interview on Jay Leno.
Taylor Lautner, Rob and Kristen also discuss what it was like to work with the actress. Check out the video below and be sure to follow TheImproper on Twitter for all the latest Robert Pattinson and Kristen Stewart updates you can trust.