Rhys Ifans didn't realise that Maggie Gyllenhaal was American until three weeks into the Nanny McPhee and the Big Bang shoot.
The Notting Hill and The Boat That Rocked admits to being stunned to learn that the Oscar nominated actress wasn't British because he accent was so good.
Speaking to Live From Studio Five the actor said: "It was amazing. I didn't realise she was a Yank until about three weeks in.
"She kept it going the whole time, even in the make-up trailer, this pristine period English accent.
"I almost wanted to spill hot coffee on her to see what it would take to get this girl to do (an) American (voice). She was that good."
Nanny McPhee and the Big Bang is just one of several projects that Ifans has awaiting release; Mr Nice and Greenberg are also on the horizon.
He is also set to team up with Mickey Rourke and Megan Fox in Passion Play before taking on the role of Xenophilius Lovegood in Harry Potter and the Deathly Hallows.
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