While Saoirse Ronan soars as the title character in “Lady Bird,” she would be nowhere without her best friend Julie, played by Beanie Feldstein. The 24-year-old actress is also currently enjoying her Broadway debut in “Hello, Dolly!” opposite Tony-winner Bette Midler. Those musical skills came in particularly handy, as Lady Bird and Julie both end up participating in their high school musical “Merrily We Roll Along.” Her comedy chops run in the family — older brother is Jonah Hill — but it’s the sweet bond that the two high schoolers share that offers some of the film’s most touching moments.

Feldstein: “My agents emailed me and said, ‘We read this script, we think it’s brilliant and the second we read Julie, we thought of you.’ Just the description of the character made my heart race, it said: ‘Sweet, chubby, gets the lead in her high school musical.’ And I was like, ‘What?’ Ten minutes after reading the script, they said, ‘We have an appointment with you to read with Greta.’ I was so excited and beyond terrified. She read the role of Lady Bird while I read Julie and we did Julie’s four main scenes. I’ve never left an audition feeling so great. I put everything I could out there and if it didn’t go my way, I’d be devastated because I loved the script, but I’d also know I’d done everything I can do.

“Two weeks later I found out I got it and I freaked out. I get emotional thinking about it, I can’t believe I’m in it. I wanted it so badly I didn’t even tell my mom about the audition, and I tell her everything. So when I knew I had it, I just called her and started screaming.

“I am a lot like Julie, though I think I’m much more extroverted. And I have a bit of the fire of Lady Bird. Julie is shy and soft spoken and I’m … a lot.

“My favorite moment in the entire film is when Lady Bird and I are singing ‘Crash Into Me’ in the car. Saoirse and I have never laughed harder than when we filmed that. I saw the film before her and texted her. I sent her paragraphs about how brilliant she is. And she wrote back just saying: ‘Please tell me “Crash Into Me” is still in the movie!’ I’m still laughing.

“My other favorite part was getting to do all the musical stuff. This is actually my dream: I’m in a Greta Gerwig film playing Saoirse Ronan’s best friend and I get to be in a musical in the movie? I didn’t know ‘Merrily’ that well before we started working on the film. So I went to the New York Public Library for the Performing Arts in Lincoln Center where they have an archive of every musical. I watched ‘Merrily’ in full to prep and just fell in love with it. Then the documentary about ‘Merrily’ [“Best Worst Thing That Could Have Happened”] came out the week I finished ‘Lady Bird’ and the day before I started the ‘Hello, Dolly!’ workshop. It felt like the marking of one experience and the beginning of another.

“Working on this film was a dream and it’s so easy to play Saoirse’s best friend. Anyone who meets her would fall in love. We’re both warm and cozy, we’re always hugging. We felt instantly comfortable together. I can’t believe I got to act with her and I learned so much. I’m a better actor now having worked with her. I truly mark my life as Pre-‘Lady Bird’ and Post-‘Lady Bird.’”