Naomi Ackie and Nafessa Williams on Portraying Whitney Houston and Robyn Crawford's 'Pure Love' (Exclusive)

Taking on the role of Whitney Houston in Whitney Houston: I Wanna Dance With Somebody was no small task, and not one Naomi Ackie took lightly at all.

"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….