Saoirse and Greta first met at the Toronto Film Festival in 2015 when Saoirse was promoting Brooklyn (2015) and Greta Maggie's Plan (2015). Saoirse had already read Greta's script and instantly connected with the titular character, so when both ladies discussed the script at length in Saoirse's hotel room and she read the key part aloud, Greta knew that she'd found her "Ladybird."
The fact that Christine (Saoirse Ronan) and best friend Julie (Beanie Feldstein) wore the same color nail varnish, was pure coincidence. And since there was a considerable degree of improvisational latitude allowed for character development on-set, a decision was made to have them wear the same varnish for the entire film.
It temporarily broke the record held by Toy Story 2 (1999) (163 reviews, all "fresh") of the best-reviewed movie of all time on Rotten Tomatoes, with 196 "fresh" reviews in a row. However, it ended up getting it's first "rotten" review after counting 197, therefore no longer holding a perfect score.
Nearly all of the songs sung in the audition scene were written by Stephen Sondheim, who also composed the musical they end up performing: "Merrily We Roll Along." The audition songs include "Being Alive" from "Company," "Everybody Says Don't" from "Anyone Can Whistle," and "Giants in the Sky" from "Into the Woods."
2nd movie in which Saoirse Ronan has played a character who insists on using a name other than their given name (Lady Bird instead of Christine). The first was How I Live Now (2013). (Daisy instead of Elizabeth)
Features the song "As We Go Along" by The Monkees, which was recorded for their 1968 cult classic "Head." Written by Carole King and Toni Stern, it was recorded at Wally Heider's, Hollywood, May 30, and Original Sound, Hollywood, August 1, 1968. Micky Dolenz sang the lead vocal, and still regularly performs the song as he continues to tour. (as of 2017)
The trivia item below may give away important plot points.
In the film, Lady Bird applies to Columbia University and later learns she was not accepted. Timothée Chalamet who plays Kyle in the film actually went to Columbia University for one year before deciding to drop out and focus on acting.