Daniel Radcliffe and Emma Watson regularly caused friction on the Harry Potter set by engaging in furious bust-ups and refusing to talk to each other.
The actor counts Watson as one of his closest friends, but admits his co-star infuriated him when they took opposing stances in their frequent debates.
He tells Britain's Radio Times, "Oh, God. We used to argue about everything. Religion. Politics. I remember one of the big arguments we had on the fourth film - we didn't speak to each other for a couple of days - was about... She was arguing about the Latin language, that nobody knows what it sounds like, what a Latin accent should be. And I was like, 'Yeah, but it's still spoken a lot in the Catholic church.'
"It got totally out of hand. She was furious; I was livid."
But the Brit admits his brainy pal often got the better of him, adding, "I certainly would advise anyone, if you're going to debate with her, know your (facts)."