The Brit stars as free-spirited teen Sam in the coming-of-age drama, which features a central scene in which her character stands up in the backseat of a car through the sunroof, and stretches out her arms as her pal drives through a tunnel.
Watson reveals a team of stunt experts wanted to hire a stand-in, but she was adamant she should be the one to film the key moment.
She tells U.K. TV show Daybreak, "It was so amazing. It was really funny, the stunt team on the film really didn't want to let me do it. It was meant to be a stunt girl but I was like, 'You can't not let me have that moment!' It was weirdly emotional. It really felt like... metaphorically like I was coming through something. It was cool."
Watson also admits playing an ordinary student gave her insight into the high school experiences she missed out on growing up on the Harry Potter movie sets.
She says, "She's this awesome character and very different to obviously what I've done before so it was really nice... I kind of got to have a bit of what I didn't have. Just the experience of making it has been really meaningful to me."