Amy Schumer still surprised she’s in ‘Trainwreck,’ the film she wrote

In her new act, Amy Schumer has a bit about how she doesn’t think she’s pretty enough to star in her own movie.

She jokes about how when she found out Universal Pictures wanted to make “Trainwreck,” the film she’d written, she assumed the studio would cast a “beautiful actress” in the lead — “like Kate Upton, or Middleton — a Kate.” And there she’d be on set, off in a corner with her “laptop and a messy bun,” looking like a “garden gnome.”

But Judd Apatow — who with “Trainwreck” for the first time directed a screenplay he didn’t write — has always been interested in upending moviegoers’ idea of the traditional movie star. For his directorial debut, “The 40-Year-Old Virgin,” Apatow chose Steve Carell as the lead even though “The Office” star had never carried a film before. In “Knocked Up,” he made it seem plausible that Katherine Heigl’s buxom career woman would go for a hairy stoner like Seth Rogen. And as a producer, he’s played an instrumental role in the rise of Lena Dunham, whose character on “Girls” once lay down in the middle of a hike and started listening to “This American Life.”

Schumer is the latest fledgling star he’s putting his might behind. For years, Apatow said, he didn’t write about his own life because he thought it was “too boring.” He’s trying to save Schumer from a similar fate — and helping her identify her comic voice seems to have imbued them both with a new kind of pride.

Before “Trainwreck” — which hits theaters in July — Schumer had never been in a movie. But she’d starred in her own Comedy Central show, “Inside Amy Schumer,” which starts its third season next month. Not to mention her extensive performance history: She’s been on the road doing stand-up virtually nonstop since she placed fourth on NBC’s “Last Comic Standing” in 2007. That her “arms register as legs” in Hollywood? (Her words.) That was of no consequence to Apatow.

“Had I gotten a phone call saying, ‘Oh, we’re going to have auditions for the role of Amy today,’ I would have said, ‘Yeah, of course,’ ” Schumer insisted.

“But my entire world is based on the fact that we never do that to anybody,” Apatow said. “That’s like having someone else sing your song before you get to sing it.”

It was the morning after an early cut of “Trainwreck” premiered at the South by Southwest film festival, and Schumer, 33, was preparing for a day of press with her director. They both looked sleepy. After the screening — which went over huge with both festival-goers and critics — they’d dropped into a local comedy club to do about 20 minutes of stand-up apiece. Apatow told funny stories about being a dad to teenage girls — and called out Bill Cosby, which he’s been doing a lot of on Twitter over the past few months. Schumer, meanwhile, stuck to material about her adventures in Hollywood — talking about how she worked with a personal trainer to get ready for the movie.

She is often scantily clad in “Trainwreck.” The first time we see her in the movie, she’s wearing just a bra and underwear, about to hook up with another random fling. This is what the Amy in the movie does: During the day, she is successful at her high-profile magazine gig; at night, she is successful at taking guys home from bars.

She says she’s repulsed by monogamy — by the life she sees her pregnant sister (Brie Larson) leading in the suburbs with a nerdy husband (Mike Birbiglia). But when she falls for a sports doctor she’s writing about (Bill Hader), she finds herself inching toward a life she’s long been so openly disdainful of.

Like Schumer, Hader was surprised to learn Apatow was considering him for the film’s romantic lead. After nine years of playing eccentric characters on “Saturday Night Live,” Hader has gone on to lend his voice to a number of animated films and has played the quirky best friend in live-action films like “Forgetting Sarah Marshall” and “The Disappearance of Eleanor Rigby.”

“When Judd said he wanted me to come in for the lead, I was like, ‘What?’ I’m just never asked to be in that part,” said the actor, 36. “You want me for the weird ex-boyfriend kind of thing.”

But soon he found himself going in for screen tests, and he and Schumer even went to a party for “Girls” together in New York, where the film is set. Apatow followed the would-be couple around the whole night, taking photos of them together and asking his colleagues if they seemed like believable lovers.

By this time, the director had already put Schumer’s script through the paces. When he first heard her interviewed on Howard Stern’s Sirius radio show in 2012, he was impressed by her honesty — the way she talked about both her sex life and her dad’s struggle with multiple sclerosis. So he asked her to write a screenplay, hoping it would have the same heartfelt voice.

It didn’t go well.

“The first time around, I wanted to write a movie that he would want to make,” Schumer explained, referring to Apatow’s reputation for making raunchy R-rated comedies.

“It was more of a high-concept comedy,” Apatow said. “But I could tell there was a more personal story in her. The most interesting thing is the most honest truth. You see Louis C.K. and you think, ‘Wow, when he revealed himself, the world embraced him.’ Nirvana reveals raw truth and millions of people go, ‘I get it. I feel the same way.’ I’m always encouraging people to strip it down.”

“He says ‘Go there, girl!’ a lot and snaps his fingers,” Schumer kidded.

“In a Z-formation,” Apatow said.

So Schumer took another stab at the script, and this time pulled material straight from her own experiences. “Not even parallels to my life,” she said. “Just, like, same lane.”

Which is why it’s weird to hear that she tried to get in shape for “Trainwreck.” The Schumer audiences know is confident in her own skin. She loves food and doesn’t relate to those girls who absent-mindedly forget to eat lunch. After a blogger said it seemed ludicrous that a woman who looked like Schumer could attract so many men, she took to Twitter to post a picture of herself in lingerie. “I am a size 6 and have no plans of changing,” she tweeted in February. “This is it. Stay on or get off. Kisses!”

Schumer said she started working out with celebrity trainer Harley Pasternak after it was explained to her how big a movie screen is in comparison to the televisions she’s used to appearing on.

“I just honestly needed to eat super clean for the energy to have long days like that,” she said. “No one’s ever been worried about me. No one has ever said, ‘You’re not eating enough.’ I still feel like a kid on the outside.”