Speaking to U.K. chat show host Graham Norton, she confessed performing the character's big number was "challenging," because she was so overcome with emotion.
"Singing 'Memory' was challenging. It's so emotional," the "Spotlight" hitmaker confessed. "I could cry or sing but not both at the same time, so I had to find a balance."
It's not the first musical role for Hudson, who notably starred alongside Beyonce Knowles in the hit film "Dreamgirls", but she reflected this time around was different as the vocals for the 2006 flick were pre-recorded, whereas "Cats" was all sung live.
"I had to sing it live 30 times," she revealed. "I could sing the song over and over, but I was drained from the tears. The emotion gets to you."
"Cats" hits theatres December 20.