The movie “Julie & Julia” intertwines the stories of two women—Julia Child discovering her love of cooking and blogger Julie Powell cooking her way though Child’s “Mastering the Art of French Cooking.” The film’s star, Meryl Streep, and its writer and director, Nora Ephron, play off their links, too. They came to know each other through their friendships with director Mike Nichols and worked together on “Silkwood” (1983) and “Heartburn” (1986). Ms. Streep shot to the top of Ms. Ephron’s casting list when they ran into each other at a Shakespeare performance in New York’s Central Park: Ms. Ephron, who is also a producer on the movie, mentioned the project and Ms. Streep warbled, “Bon Appetit!” The movie opens nationwide Aug. 7. Columbia Pictures, which spent less than $40 million making the movie, has been building buzz among foodies. In May, the studio played a clip at the James Beard awards and it has been holding screenings for chefs and restaurateurs in major markets. “Julie & Julia” also cast several food writers as extras. Ms. Ephron, 68 years old, and Ms. Streep, 60, recently taped publicity segments at Stone Barns, where New York chef Dan Barber has a farm and restaurant in suburban Westchester County. Punctuating their conversation with raucous laughter, Ms. Streep and Ms. Ephron spoke about towering women—Ms. Streep wore platform shoes in the film—and facing off against fanboys at the box office.

The Wall Street Journal: The movie in large part is about two women discovering themselves.

Ms. Ephron: One of the things that I identified with so much was that Julia Child didn’t really become Julia Child until she was almost 50 years old. I think that thing that women sometimes get to do, which is to reinvent themselves at a certain point in their lives, is something that has happened to me a lot.

Ms. Streep: I thought of myself as a costume designer. That’s what I took my degree [at Vassar] in, my thesis project and everything was in costume design.

Ms. Ephron: It was? Really!

Ms. Streep: Even at Yale, I signed up for the LSATs. … I thought I’d save the world somehow doing environmental law. I was besotted with Jonathan Schell’s book, “The Fate of the Earth.” Slept through the LSATs because I had a show the night before. I thought it was a sign.

How did you prepare for Julia Child?

Ms. Streep: The voice and posture. I found the most valuable source was the very first cooking show, which I looked at over and over. Nora gave me all of them and I looked at the other ones. The later ones weren’t as sort of revealing of who the woman was. But that very first show… She had not one ounce of camera-consciousness, as opposed to self-consciousness. She had a lot of self-consciousness. And I just found it so touching and sort of, I could find her there.

And the platforms?

Ms. Streep: I just thought it was really important that she be very, very, very tall. Just pretend that you’re you right now, and you’re 6’2″ and your sister is 6’4″. And everybody else in the world, every other woman in the world is not. … Your physicality changes how you behave. I know in my life, when I was first entering this business, coming into the room and having long blonde hair and a long face and sort of serious mien, is that how you say it, that defined me for many, many people. It was just very hard to break out of that.

Last summer you were up against “The Dark Knight.” This summer it’s “G.I. Joe.” Who’s going to win?

Ms. Ephron: I don’t think there’s any question who’s going to win.

In terms of—

Ms. Streep: Fanboys.

Ms. Ephron: You know, you can’t outdraw that but—

Ms. Streep: We will outlast them.

Ms. Ephron: Yes, we will outlast them. That is exactly right. … They’ll have a big first weekend and we will be playing ‘til February.

The studio moved the opening from April to August because they thought the film would make more money then.

Ms. Streep: There’s a gigantic audience for women. There just isn’t stuff available to them year-round. They aren’t marketed with the same vehemence as the other products that are issued in the marketplace. But there’s certainly a gigantic market for films that appeal to women. And it always takes Hollywood by surprise.

Ms. Ephron: Every single time.