"I watched the film for the first time with my best friend in October. We had a couple of glasses of white wine before we went in because I was so nervous, but we had our arms wrapped around each other," Ronan said
"Even though I knew what was going to happen in the film, I was laughing one moment and sobbing hysterically the next. It's about the beautiful struggle of being a teenager, growing and changing and getting to know yourself, and having the realisation that relationships change," she added.
Ronan also said that she identified a lot with her character in the film.
"I identify with a lot in 'Lady Bird'. The great thing about how you've written the character is that she doesn't know how great or special she is. And there's a huge sense of the importance of belonging, belonging to a place, a time in your life," Ronan told Gerwig.
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"Lady Bird" has been nominated for five Oscars, including Best Actress for Ronan and, Best Director and Best Original Screenplay for Gerwig.
The movie has already won two Golden Globes.