Reese Witherspoon and Matthew McConaughey reveal their real spirit animals

Reese Witherspoon has a secret spirit animal.

Witherspoon told BUILD Series her spirit animal is a squirrel and Matthew McConaughey, a penguin, when they visited to dish on their Illumination film, "Sing" out in theaters on December 21.

McConaughey stars as a koala named Buster Moon in the animated picture that takes a nod from singing competitions like "The Voice" and "American Idol," in the hopes of saving a theater. Witherspoon's character, a pig called Rosita, is one of the contestants vying for a spot in the contest. While she received an Academy Award for her performance as June Carter Cash in "Walk the Line," Witherspoon found it challenging to sing Katy Perry and Taylor Swift in the animated flick.

"I would sing Katy Perry and Taylor Swift in my car on the way to the studio, and then I would get there, go into my booth and they would ask me to come back tomorrow, or if I was available for the week," Witherspoon spilled. "The songs are really challenging and it took me about 19 sessions to get three songs right."

McConaughey would gather material for his character by going to other animated movies with his kids and seeing how the kids in the theater would interact with the the animation. McConaughey says the trick to recording an animated movie for kids is thinking like an 8-year old.

For the stars who have young children, working on an animated movie was really special because their kids were finally able to see one of their films. McConaughey cited this as one of the reasons he participated in the project.

"Well, I had been looking for some voiceover work, but I got tired of answering the question: What is your kid's favorite movie of yours?," McConaughey revealed. "They [his kids] like the movie a lot, and my daughter says I am her favorite character, but I would say that overall being the film has helped me out in the household."

For more of the BUILD Series interview with Reese Witherspoon and Matthew McConaughey click here.