If you're not sure which part we're referring to, the Vicar of Dibley star, 66, appeared in Harry Potter and the Prisoner of Azkaban as the incredibly vocal Fat Lady in the painting on the Gryffindor common room door.
It was his responsibility to let students in with the password and keep those who didn't have it – like the movie villain Sirius Black – out.
When confronted by Black, played by Gary Oldman, the French legend and Saunders fled to another painting of Hogwarts – which happened to be home to a hippopotamus.
Many viewers would naturally assume it was a fake hippo. CGI, maybe. Oh no, it was a real hippo, as Dawn discovered when she arrived on set only when she was asked if she minded working with animals.
What followed was great confusion as Dawn was warned by the animal's keeper that the beast might try to smell her private parts and mate with her.
She was told to watch out for the hippo's skin turning foamy, which is a sure sign that the wader was feeling a little excited – which left Dawn 'genuinely terrified'.
Fortunately, there was no foam in sight and the hippo didn't try to mount poor Dawn. However, he smelled her private parts.
James Acaster and Ed Gamble were left in hysterics when Dawn broke the bizarre story on the Christmas episode of Podcast off the menu. But there was a surprising twist in the story that no one saw coming. Not even Dawn.
She told the comedians: 'I was asked if I was happy working with animals, and then the director told me there was a hippo. I just said yes because I wanted to be in the movie. I didn't think about it until the day… when I was left in a pen with sawdust.
'He said they were going to bring a living creature. “Pay respect, stay still, don’t run, understand?” He then explained that the script called for a female hippo, but this was a male hippo.
'Then he said, “This hippo might want to smell your private parts. Don't run.” I was hesitating because I thought, I don't think that's part of the contract. When you said, “You like working with animals, I wasn't imagining that.” That's awesome.
'Then he said, “This hippo might want to mate with you, and if he does, his skin will start to foam. Stay alert, pay attention to this, tell me, but don’t run.” Then I thought, “Oh, God, I wish I could run now.”
However, as soon as Dawn saw the size of the animal, all her fears disappeared.
'It was the size of a Labrador!' she joked. “The word that everyone stopped using in relation to the hippopotamus was pygmy. It was a pygmy hippopotamus.
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