The Harry Potter star was given tickets for the EdgeWalk, which enables thrill-seekers to take a hands-free stroll on top of the main pod of the 553-metre (1,815-foot) high landmark in Toronto.

The surprise was a birthday gift from Alejandro Amenabar, the director of Regression, which Emma was shooting at the time, but she only agreed to take part in the stunt if he joined her.

"Alejandro the director, as my birthday gift, bought me the CN Tower walk, which is one of the tallest buildings in the world, and they put you in harnesses and you can do this walk on the top," Emma tells U.K. TV show This Morning.

"What he didn't realise when he gave me the present was I was going to force him to do it with me. I think with foresight he might have given me a different gift."

Emma began shooting her scenes for the thriller, in which she plays a sex abuse victim, on her 24th birthday in April last year (14) and she took to to tell fans, "First day shooting Regression today - a very cool birthday present."

Now she has also revealed how the timing of the shoot meant she received an unusual welcome on the set, adding on This Morning, "Everyone sang me Happy Birthday!"