The heavily scrutinized relationship is explored in the upcoming biopic, 'Whitney Houston: I Wanna Dance With Somebody.'
"I was in prep for about six months, seven months, by myself, and with my dialect coaches," the London-born actress told ET's Rachel Smith at the Whitney Houston: I Wanna Dance With Somebody Experience in New York City over the weekend.
Adds Ackie, "I had three amazing dialect coaches, an amazing movement coach called Polly Bennett" -- who also worked with Oscar winner Rami Malek on Bohemian Rhapsody and awards season frontrunner Austin Butler on Elvis, and helped Ackie embody Houston both onstage and off.
"I'm a slouchy girl and Whitney was very poised," she added with a laugh. "It was that, studying the songs and her history and what she was like as a child and all this beautiful, imaginary work that me, as an actor, I just like live for... It really pushed me to my outer limits and I'm so happy that I got to do it."
A major part of Houston's life that Ackie dives into in the film was her relationship with her best friend, Robyn Crawford -- played in the movie by Black Lightning star Nafessa Williams -- which was cast back into the spotlight in 2019 after the publication of Crawford's book, A Song for You: My Life With Whitney Houston, which detailed the pair's romantic and physical relationship in the early 1980s.
In her book, Crawford writes that it was Houston -- who died in February 2012 -- who ended the physical part of their relationship in 1982.
"She said we shouldn't be physical anymore, because it would make our journey even more difficult," Crawford writes, adding that Houston gifted her with a Bible when she told her this. "She said if people find out about us, they would use this against us, and back in the '80s that's how it felt."
"Me and Nafessa, we talked about how really, the basis of their relationship is a friendship," Ackie recalled of portraying the pair onscreen. "It's love, it’s a strong, strong bond, and we just wanted to show how much they sparked off of each other and loved each other and joked around and were able to achieve so much because of the support they had in each other -- especially Robyn supporting Whitney."
"What they had was gorgeous, and we just wanted to represent it properly and give a nod to that love that they shared," she added. Houston was later married from 1992 to 2007 to Bobby Brown, who is played by Moonlight star Ashton Sanders in the biopic.
Williams said that Crawford's book was her "saving grace" when it came to playing a real-life person and being "authentic to who that person is."
"I felt like everything I needed to know about her in life lived in that book," the actress noted, adding that she also created themed playlists to take her back to the '80s and '90s, musically.
"This isn't the Whitney Houston movie that you think you're coming to see," she added. "We're about to learn more sides of who she is. For me, my priority was to the responsibility that they had to one another in their friendship, to showcase that, whether it was romance or friendship, it was just pure love and loyalty."
Another source of love and loyalty in Houston's life, of course, was her mother, Cissy, played in the film by Tamara Tunie.
"We had a great time with Cissy," director Kasi Lemmons told ET, with Tunie agreeing and noting, "It was just a matter, for me, of just trying to be as honest and authentic as I could be."
"It’s not an impersonation," she continued. "It’s about finding the truth of being a mom of an icon, who was a legend in her own right."
"Cissy was her mentor and her first teacher," Lemmons noted.
To prepare for the role, Tunie said she "read everything, watched everything, had conversations with people who knew Cissy and experienced her... That was all very helpful, and I was just really hoping to tell the truth of the relationship between this mother and this daughter, at the end of the day."
"Clive was amazing," Lemmons raved. "I mean, first of all, it gave me a lot of confidence going in that people that were involved in her life and her relatives and people that worked with her closely were gonna be involved in the film, they were gonna be resources to me."
"I can call them on the phone, I can hear what they had to say... I sat in Clive's house and watched videos, movies of Whitney that he had, that most people don't have access to, so that was an enormous resource," she added.
For Ackie, the positive feedback from Houston's family and famous friends has come "as a beautiful surprise."
"I appreciate them for listening to my creativity and having an input," shared the actress, who is also an executive producer on the film. "For me, it's a sign that they really loved what I was adding to it... It's helpful to me, because that's what I want to do in the future. I want to be able to provide storytelling platforms for more people."
"Reading what Clive had written, what Pat [Houston] had written, it was so heartwarming," she added. "And to get to see them in so many different parts of the process, they were always so supportive. Always willing to share anything to help me tell the story properly. I'm eternally grateful for them, and really quite touched by their involvement and how much they wanted this part of her story to be told."
The actress also noted that sharing the film with the world feels like the "final piece" in her journey with Houston, adding that she can't wait for fans to watch it and "reignite their love for Whitney."
Whitney Houston: I Wanna Dance With Somebody is in theaters Dec. 23.