Wittrock, an American Horror Story favorite recently seen in American Crime Story: The Assassination of Gianni Versace, will play Garland's fifth husband, Mickey Deans. Also joining the cast is Jessie Buckley, who will play production assistant Rosalyn Wilder.
As previously reported, Oscar winner Renée Zellweger will play Garland in the film, which focuses on the entertainment icon's final London concerts.
The project will be directed by Tony nominee Rupert Goold (King Charles III) based on a screenplay by Tom Edge (The Crown). Filming is expected to begin in March and continue through May. A release date has yet to be announced.