(CNN) -- There's a moment, when singer LP performs the song "Into the Wild," that her face tightens and she purses her lips, even as she keeps singing. It happened show after show at the South by Southwest music festival, whether on sunny daytime stages, in dark clubs, or while rocking a ukulele, solo, in the middle of an Austin, Texas, street. She is trying not to smile.
She knows the line that will make people recognize the song, and its singer. Maybe they saw her belt it at Sayers Club in Los Angeles, or they're among the millions that watched it on YouTube, or they remember how it burst from a Citi commercial as a rock climber ascended a 400-foot sandstone tower: "Somebody left the gate open, you know we got lost on the way."
Those big notes give her away: Not Laura Pergolizzi, songwriter, but LP, the singer crowds have heard, but never heard of. The artist, whose CD/DVD, "Into the Wild," debuts Tuesday; who'll be on "Jimmy Kimmel Live!" on Wednesday; and whose album with Green Day producer Rob Cavallo will come out later this year.
Always after that line, she knows, the murmurs start.
"I see a few of them, every time, look at their friend and go, 'Ohhhh.' It's kind of funny and embarrassing at the same time," LP said during an interview at SXSW. "And awesome."
The singing, the recognition -- they weren't part of any big plan. Pergolizzi's dad was a lawyer. Her mom sang opera, but gave it up when she had a family. Growing up in New York, they listened to oldies, not pop radio. But after her mom passed away in 1997, Pergolizzi finished high school, moved to Manhattan and found music was what made her happy.
"I didn't know anybody, I didn't have any connections," she said. "The first time I wrote a song, I couldn't really believe -- 'Can you just do that? You're just allowed?' I never thought about songs on the radio, and who wrote them."
She was first signed as an artist on Island Def Jam, then RedOne. The deals never shook out, but she gained a reputation as a writer who could churn out dozens of tunes to suit any voice or sound. She wrote Rihanna's "Cheers (Drink to That)" and chunks of Heidi Montag's album, "Superficial." Christina Aguilera and Backstreet Boys cut her work. There are songs in the works with Joe Walsh, Rita Ora and Isa Summers from Florence and the Machine.
Writing was so demanding that she left New York in 2010 and moved to Los Angeles. She'd show up to sessions with a ukulele -- a $60 instrument she'd taught herself to play using Beatles songs posted online -- and strung together melodies and lyrics that worked.
Creating pop for other people made her understand what songs need, she said, but as an artist turned writer, she was compassionate. She understood what it was like to sit in a room with people trying to decide your voice and put words in your mouth.
"I was having a nice anonymous little time as a writer," she said. "I really was on the writer path. I was sort of minding my own business. I loved making a living in music."
Sometimes, for fun, she'd show up at Bardot, Soho House or Sayers Club in Los Angeles for one- or two-song cover sets -- "Barracuda" by Heart, "The Funeral" by Band of Horses, "Uninvited" by Alanis Morissette or "Where Did You Sleep Last Night," the Lead Belly version Nirvana remade.
The crowds at music industry hangouts always liked her; she was one of them. One night, when it seemed right, she pulled out her ukulele and played "Into the Wild" on stage. The audience exploded. She remembers people chanting her name, once adopted just for convenience, later because it felt right: Not Laura -- LP.
"When I'm behind a big song, I feel like I can do big things," she said.
That moment was a big one. She signed to Warner Bros. Records (which is owned by CNN's parent company, Time Warner), found managers she trusted, and began to think more about what her sound should be -- not what others wanted, or what other singers needed, but what hit all the highs and lows in her vocal range and personality.
"The thrill of writing songs for other people, when you get that right, that person's soul speaks to that song, you've done them a favor and the world a favor," she said. "That's what writing is all about -- you're always trying to get the real picture.
"I wrote a bunch of songs I feel only I can sing."
It's not an anonymous life now.
For long days at SXSW, Pergolizzi had managers, PR people, photographers and a band around her. The singer had a signature look -- a pillow of tight black curls around her face, broken-in black boots, simple silver jewelry. She also had about seven black jackets that all looked more or less the same. She looked on the outside like she feels on the inside, LP said, and finally has the instruments and stage to express it.
Crowds piled in to see her in Austin, or stopped to listen when they heard the line, one they all recognized. They'd always look up to see the person it was coming from.
Maybe it all looks like a character, LP said -- the new city, the name change, the uniform, the ukulele. But she knows now which songs are hers.
"I think everybody needs to find out who they are and who their, like, inner superhero is, or who you like to be," she said. "The world would be a better place if everyone was doing what the hell they wanted to do and being who they wanted to be.
"I hope they do."