According To Anne Hathaway, This Iconic Princess Diaries Moment Was Completely Unscripted
In fact, if you look closely at the scene Anne Hathaway describes you clearly can see that it was totally unscripted and it was more like an accident

The Princess Diaries (Disney)
Want to feel old? The Princess Diaries was released twenty years ago. Based on Meg Cabot's 2000 young adult novel of the same name, we got to see a younger Anne Hathaway, Heather Matarazzo, Mandy Moore and how could we forget the amazing Julie Andrews?
In fact, some weeks ago the actresses were celebrating the 20th anniversary of the film on social media, making us feel all nostalgic, however, this time, we would like to talk about that time Anne Hathaway revealed that one of the memorable scenes in the movie was in fact, not rehearsed and all-natural. This revelation was unveiled last year when Anne Hathaway contributed to The Happy Days of Garry Marshall, a special honoring the film’s legendary director, who passed away in 2016.
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“One of the things that I love about Garry is that he was never afraid of the spontaneous moment,” Hathaway said in the clip. “If it was good, it was in the movie. He didn’t care where it came from; he didn’t care what happened.” The perfect example was this scene we are talking about, where Hathaway fell on the bleachers while shooting a scene with Heather Matazarro, who played Lily.
“And there was this moment when we were filming,” Hathaway explained. “We were filming in San Francisco. Predictably, it had rained. We were outside, and we had to get the shot really fast because we were losing light. And we had to go on these bleachers and they did their best to wipe them down. And Gary had just said, ‘Okay, there’s a lot of exposition in this scene,’ but he would’ve said, ‘There’s a lot of pipe in this scene, so we just have to say it really fast.’”
Marshall told Hathaway that “whenever Heather Matarazzo is at the bottom step, I want you to be at the top step of the bleacher. And you just walk back and forth, and we’re gonna get all this information and it’s gonna be cute because you guys are moving.” The result? Not what the director asked but instead it was an amazing and spontaneous scene that made it to the film. It looked like it must have hurt quite a lot and even bruised, but bot Hathaway and Matarazzo keep playing and improvised from there.
“I guess one take Heather had a lot of energy, so she was moving faster, and I mistimed it, and I realized that we were going to be on the same step, so I turn really fast, slipped on a puddle, and just fell,” Hathaway explained. “Absolutely fell and burst out laughing and kept going with the scene. Never thought about it again and all of a sudden, like six months later, it’s in the trailer. He kept it! He kept it because it was a charming moment.”