The multi-instrumentalist revealed Disney Pixar, the makers of the film, asked her to perform the songs despite never actually having heard her sing in English.
Fowlis said she barely knew herself what she sounded like in the language, as she almost always performs in Gaelic, adding it was such a different style that it was like acting.
However, she is delighted the movie's trailer features her singing a Gaelic song, bringing the language to a worldwide audience of 40 million.
You can listen to the song, Tha Mo Ghaol Air Aird A' Chuain (My Love is on The High Seas) by clicking on the icon in the box above.
Brave made its debut at number one in the US box office chart, making an estimated $66.7million (£42.8m) in its opening weekend last week.
It is predicted the movie will bring a £140m boost to Scottish tourism.
Stars, including Billy Connolly, Emma Thompson, Julie Walters, Kelly MacDonald, Robbie Coltrane, Craig Ferguson and Kevin McKidd, all speak the parts of different characters in the film.
Fowlis attended the world premiere in Los Angeles and is due to perform at the European premiere on the closing night of the Edinburgh Film Festival on Saturday, ahead of the film's Scottish release on August 3.
In an interview yesterday to promote this year's Hebridean Celtic Festival in Stornoway –where she will perform on July 14 – Fowlis revealed how the call to work on the animated movie came out of the blue from Tom Macdougall, Disney's vice-president of music.
At the time she was eight-and- a-half-months' pregnant with her second daughter.
"While they were doing their research, they listened to a lot of music and picked up a lot of albums, but they said my album kept coming to the top, which was fantastic," she said.
"I was thrilled because they have the choice of any number of singers.
"I also had to tell them I was due to give birth around the time of the recording, but they were very supportive and changed their schedule to allow me to get the vocals done before the baby arrived."
Fowlis and her husband, musician Eamon Doorley, worked with the Disney Pixar US production team and musicians on both sides of the Atlantic in the recording of two original songs for the movie, written by Pixar's Alex Mandel, to reflect the feisty main character Princess Merida.
"They were encouraging me to get inside the mind of Merida [the animated heroine], to try to encapsulate her spirit and her inner thoughts", she said.
"They are very much Disney songs, but we were encouraged to have our input, and so there is a flavour of home in there too."
The two songs are sung in English. But Fowlis, who grew up in North Uist, normally sings in her native language.
She said: "It was a real vote of confidence to ask me to sing the songs in English because they had only heard Gaelic material until then.
"It was a bit of a risk as they didn't know what I sounded like in English – I barely know myself.
"It was a bit of a learning curve because not only was the style different to what I'm used to, it was like acting rather than singing.
"So it was a challenge, but I really enjoyed it and it was great fun to do. I can tell my children when they get older that their mum and dad worked on a Disney film."
At the time Fowlis was recording her vocals for the soundtrack she also received a call from another part of the Disney organisation asking if she would contribute to the trailer – unaware she was already involved in the film.
Her song Tha Mo Ghaol Air Aird A' Chuain (My Love is on The High Seas) from her 2005 debut album Mmar a Tha Mo Chridhe' (As My Heart Is) featured on the TV advert for the film, premiered during the Oscars in March and was seen by a worldwide audience.