The 34-year-old actress got engaged to the record producer, 68, at the beginning of last month following less than a year of dating, but she has admitted she was totally unaware that he was so successful in the music industry when he saw her audition for 'American Idol', in which he was a guest judge, back in 2006.
Speaking on the 'Everything Iconic With Danny Pellegrino' podcast, she said: ''I grew up listening to Céline Dion and I love Andrea Bocelli -- I knew 'The Prayer' because I'd sung it for something and my mom made me learn the Italian.
''I remember calling [my mom] and I was like: 'You're never going to believe, the guest mentors this week is Andrea Bocelli and there's this producer that I don't know but he's supposed to be really amazing. His name is David Foster.'
''My mom was like, 'Katharine, you know who David Foster is. You've listened to every artist that he's ever created.' And I was like, 'Ohh... okay.' ''
Katharine - who was praised by David at the time - came runner up on the show to Taylor Hicks and she and the Canadian musician went on to work together.
The blonde beauty did a few live performances with David, including his 2008 PBS tribute concert 'Hit Man: David Foster & Friends'.
Speaking of their bond, she said: ''We met on that show and then I did a lot of charity events with him. We've been friends for a long time. And.. yeah...''
At the end of last year, Katharine revealed she had been friends with David - who is the ex-husband of reality television star Yolanda Hadid - for years before their relationship became romantic.
She shared: ''We're very close friends, and we've been friends for a long time. I'm really, really fond of him, and I think he's an incredible person. I've known him since I was 21 years old, you know? He produced my first single. So he's been really good to me. People can say whatever they want.''
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