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In fact, he even couldn't stop sniggering during Dumbledore's funeral.
Speaking during the recent virtual Harry Potter reunion, put together by Draco Malfoy actor, Tom Felton, Rupert confessed he always used to laugh at the most inappropriate times on set.
Check out a clip of him admitting it below:
"I remember on Potter it would always be the most inappropriate scenes, like Dumbledore's funeral was a particularly bad one," he recalled.
"For some reason I found that absolutely hilarious. And yeah, once you start laughing, it's very hard to stop."
Revealing he even earned himself a nickname because he had to refilm so many scenes due to his laughter, Rupert added "I had a particularly bad reputation, they used to call me Go Again Grint because I could never do anything without doing it like 20 times."
It wasn't just the Dumbledore scene, though. Rupert added that he and Daniel Radcliffe had to film the first scene's train carriage scene apart from each other so that they couldn't catch eye contact and get the giggles.
The reunion also saw Harry Potter actor Daniel Radcliffe join the gang, via video call, for a catch up, alongside Jason Isaacs (Lucius Malfoy), Bonnie Wright (Ginny Weasley), James and Oliver Phelps (Fred and George Weasley), Evanna Lynch (Luna Lovegood), Mark Williams (Arthur Weasley) and Alfred Enoch (Dean Thomas).
It was being held to mark 19 years of Harry Potter, but Daniel teased a bigger get-together could well be on the cards for the film franchise's 20th anniversary, next year.
"My first thoughts are that this is but a mere preview of the nostalgia we'll be immersed in next year when it hits 20 years since the first film was released," he said.
"I feel like we'll be doing a lot of reminiscing probably in various forms next year. It feels insane that that much time has gone by. It seems impossible that all that time has passed."
Can we just fast forward to next year please? A big Hazza P reunion is exactly what we need to look forward to once Covid-19 blows over.
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