The "Bridget Jones's Diary" star took on the role two years ago, and took it upon herself to immerse herself in the world of the tragic screen icon and singer, who died from an accidental barbiturate overdose at age 47 in 1969.
"I'd listen to her music every day," Renee told Britain's Daily Mail newspaper. "I'd watch footage of her before I went to bed. She was always there, always present. I had to get myself into Judy's environment: her voice, her speech patterns, the moments of vulnerability in her life that make you cry."
The actress added she spent "time with all of the information left" behind about the star, including recordings and video, to try to understand "the essence of who she was".
"She was vulnerable, always hopeful. Funny, naughty, smart. And I saw there were a couple of hurdles that were just too high for her," she added.
Judy focuses on Garland's troubled five-week stint at London's The Talk of the Town nightclub, now The Hippodrome Casino, in 1968, as well as the personal struggles that began after she became a child star in the 1930s.
And Renee admitted that she empathises with many of Garland's experiences, as she has also had to deal with the downsides of fame for more than two decades.
"There are certain things about her (Garland's) experiences that I might understand," she explained. "The misconceptions about being a public persona. I've had my share of misrepresentation with the press. It comes with the deal, doesn't it?"
"Judy" will debut in cinemas on 27 September.