The odd couple teamed up in the studio to rework Frank and Nancy Sinatra's Something Stupid - and Buble admits he had to convince the actress to work with him.
He tells Rolling Stone, "I've always loved the song, and when I started thinking about the concept of how to do it the first thoughts I had were pop stars - Katy Perry, Gaga, anybody. I was just throwing out names to my manager, and he's the one who said, 'What about Reese Witherspoon...?' I told him, 'It's an incredible idea, but good luck getting her.'
"Lo and behold he called (her talent agency) Caa, we got in touch with her husband, and I ended up getting on the phone with her. She was understandably nervous - it's not her world. I told her I was gonna take care of her and if it wasn't right and didn't come out the way she hoped it would, I'd scrap it.
"She came into the studio, she was prepared, and it was nice for me, because she was one of the few people who I've ever talked about the song with who understood the premise of the song so well."
The track was previously covered by Robbie Williams and Nicole Kidman.