He tells Britain's The Sun, "The album is designed as a whole piece. It has boy and girl lead characters and top of our list for the girl part was Rihanna ... but it took a while to pluck up the courage to ask her."
"We did a show in Las Vegas and I met her and said, 'Do you think there's any chance?' I was very Hugh Grant-like and spluttering about it," he explains. "She has such an amazing voice and it's so different to mine. When the vocal happened I thought 'Wow, you sound different,' but I think that's also because she was singing a different song to what we are used to hearing from her."