The Real Steel star said they met up after a flight and ended up going out together for a night out.
He said to MTV News: 'I actually did karaoke with Rob. That's random, isn't it?
'We were on our way to Japan, and he was promoting [Twilight] and I didn't talk to him at all on the flight, because he's this really tall guy with a hood over his head literally, and every time I'd go to the bathroom, I'd be like: 'Man, that guy takes some serious sleeping pills.'
'Because this was him [slumps over] the whole time.'
He added: 'When he got out, I was like: 'Hey, man.' And Baz Luhrmann was also on the flight because we were promoting Australia. So we went out to his Japanese karaoke bar, and we invited Rob, and he came.
'And it was so much fun. Man, he's got a voice. Really soulful.'