Appearing on the Jonathan Ross Show with his Gunpowder Plot co-star Liv Tyler, the recently-engaged actor shared footage of the moment his prank took place.
It showed Leslie opening the fridge in their home, spotting something then screaming and collapsing on the floor.
The "something" in the fridge was revealed to be a replica of Harington's head - perhaps made by one of the makeup artists on Game of Thrones.
"It didn't go down well," Harington said, explaining that while his family regularly participates in April Fool's, Leslie's family does not.
"After that she was in tears," he said. "It didn't go down well. She pretty much told me if I did it ever again that would be it - and I think that's marriage included."
Ross asked him if the couple were planning on a Game of Thrones style wedding.
"There is absolutely no chance of me convincing her of that," he replied.
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