Well, that's what she told Telegraph.co.uk website in an interview on her singing debut.
'I just knew how fantastic it sounded in the shower,' she confidently told the website.
The Lost In Translation star and Playboy's sexiest girl alive debut album is titled Anywhere I Lay My Head and will be released on May 19.
Featuring a selection of cover versions of Tom Waits songs with David Bowie on back up vocals, Scarlett's record, according to the website, sounds like a bizarre mix of a hand-wound music box, massed banjos, birdsong and the choral chirruping of detuned crickets.
As for why she settled for Tom Waits covers, the actress said, 'I have absolutely no idea why. I just love his songs.'
All the weirdness notwithstanding, the sultry actress -- who has appeared in videos for Bob Dylan, Justin Timberlake, and recently Will.i.am's video for Barack Obama -- is pretty confident of her singing abilities.
'I don't follow pop," says Johansson. "I don't really know what's in the charts. I don't need to be J-Lo, I'm not trying to make a hit record and have a second career opening ice rinks at the Rockefeller centre. I don't feel I have to prove anything to anybody. I am over the moon, deeply moved by this album, honoured to have had the chance to make it. It's something I love, and I love to share it with people.'
Photograph: Getty Images