"Frozen" was the biggest animated film of all time created by Jennifer Lee and Chris Buck. Find out why they are so apprehensive on making the follow-up, "Frozen Fever."

Disney's 2013 film "Frozen" is the highest grossing animated film in history. It also won Oscars and owned the Billboard chart. In April 2014, the studio approached the co-creators and directors Jennifer Lee and Chris Buck to create a "Frozen" short film, but instead of being excited, the duo was hesitant.

"It was the scary thing of, 'How do you follow 'Frozen' up?'" Lee shared to BuzzFeed. "We were scared. I don't think we even began to understand what people liked about the movie."

"I don't think you can ever quite get your head around it," Lee went on. "And just when you think you're starting to figure out why, you realize you have no idea. You go out to dinner and there's kids wearing a 'I Like Warm Hugs' T-shirt and it hits you again. Two years ago, we were writing that and wondering if kids would even like this character. Every day we're still surprised. I don't know if I'll ever understand it."

"To have something like this take off is unreal," Buck shared to the conversation. "And to see it and hear about it all the time, you kind of pinch yourself. It's kind of an out-of-body experience. Like, 'Whoa. We did that.'"

After some time, Lee and Buck agreed to brainstorm at least, some ideas that could possible make-up "Frozen Fever." The early pitches mostly centered around Olaf.

"It just felt like he was perfect for a short," Buck said. Eventually though, the team realized that the short had to be about "Frozen's" most popular angle, the relationship between the sisters Anna and Elsa, especially now that they could have fun with the curse-free sibling.

Head story artist Mark Smith finally pitched in a very simple yet interesting idea: "What if Elsa had a cold?" And so "Frozen Fever" was created.

"Frozen Fever" will debut before Disney's "Cinderella" on March 13 in theatres.