Actress Greta Gerwig repeatedly messed up her lines on the set of Maggie's Plan because she was so in awe of co-star Julianne Moore.

In the new romantic comedy, the Frances Ha star portrays the titular character, a woman whose plans to become a single mother are derailed when she falls in love with a married man, played by Ethan Hawke.

Julianne features as Ethan's onscreen wife, and Greta confesses she became so engrossed in watching the Oscar winner at work that she frequently missed her own cues, much to her embarrassment.

"She's tremendous, as is Ethan, and everyone in the movie's great," Greta gushed to breakfast show Today. "She plays this Danish, imperious professor, who seems very cold but is actually kind of a mess herself.

"She's so funny and I think people don't think of Julianne Moore in comedies, but she's so funny in this movie and she has such impeccable timing, and it was very intimidating to work with her, but working with great actors just makes you good.

"I would watch her - and Ethan too - I would watch them act and was just so drawn in by what they were doing, I'd forget that I had a line next. It's so bad!"

The 32-year-old will be hoping the cast in her directorial debut, comedy Lady Bird, won't have such problems when production begins on her first feature film behind the camera. Greta admits she has been suffering sleepless nights about the daunting task.

"It's super cool. I'm really excited and really nervous," she confessed. "I have terrifying dreams about it every night! I had a dream last night that I didn't have a monitor to look at the shots... So I'm excited and scared!"

Lady Bird will star Saoirse Ronan as a young woman who explores new adventures while living in Northern California.