The British actress has had to dress up as a variety of characters for her varied film roles such as terrifying headmistress Miss Trunchbull in 'Matilda' but it was the skin-tight outfit she had to don to play Aunt Marge in the third instalment of the wizarding franchise when the character was blown up like a balloon in the 2004 film which was the most uncomfortable.
In an interview with the Metro newspaper, Ferris, 68, has revealed she struggled to breathe when shooting those scenes.
She said: "I had several suits of latex which they put compressed air between to expand me. It was absolute torture. And I passed out. It had never been done before, you see.
"We found that if we fired the air in, it stopped my lungs expanding so I couldn't breathe, which is why I passed out. So we had to have a hard carapace underneath. I was so big I couldn't get through doorways so I had to change in a tent."
And being kept separately from the cast is something that she had to do on the set of 1996's 'Matilda' - based on Roald Dahl's novel of the same name - which starred Danny DeVito, who also directed.
She explained: "Danny tried to keep me away from the little ones in the cast because he wanted their real reactions, so they were all a bit wary of me. But their curiosity overcame them. I'd be there looking terrifying and I'd feel a little hand slip in mine. We became close friends."