The duo will portray the roles of Queen managers in the fim directed by Bryan Singer, according to The Hollywood Reporter.
Gillen will play John Reid, rock band's manager from 1975-78, while Hollander will play Jim Beach, Queen's longtime manager, who took over in 1978.
The actors are the latest additions to the cast, led by Rami Malek who stars as Queen frontman, Freddie Mercury.
Other cast members include Roger Taylor, Joe Mazzello, Ben Hardy and Gwilym Lee, who are playing Queen bandmembers, as well as Lucy Bolton who essyas the role of Mercury's life- long companion, Mary Austin.
"Bohemian Rhapsody" is slated to release in the US next year on December 25.