The Hangover star has been dating the blonde beauty since meeting her last year (Feb13) at the U.K.'s ELLE Style Awards, but they have both stayed quiet about the relationship and only confirmed their couple status in January (14) by stepping out at an event as a twosome.
Waterhouse has now spoken about how the pair got together, revealing she was blown away by Cooper's dancing.
She tells Rollacoaster magazine, "I met him at the Elle Style awards in London last year. We were introduced and hit it off almost immediately. We were dancing at the after-party, and he asked me if I fancied going to a club. We went to Cirque Le Soir in London - and he's a ridiculously good dancer, yes. But I'm a serious dancer too, so I don't know if I was fully concentrating on that. We make a pretty good duo, though."
Waterhouse adds of her decision not to speak about the romance until now, "I never get asked about it, because they (journalists) all think I'm not going to answer them."