Fillion's role in the film is being kept under wraps. The actor, who currently stars on ABC's The Rookie, has appeared in other Gunn projects such as Slither, and Super.
Agee will be providing the voice of King Shark, a humanoid, villainous shark who has been a featured member of the Suicide Squad in the pages of DC Comics.
The Suicide Squad is set to hit theaters on Aug. 6, 2021, with production set to start in September. Gunn, best known for helming Marvel's Guardians of the Galaxy series, penned the script.
Margot Robbie stars as Harley Quinn, Viola Davis as Amanda Waller, Jai Courtney as Captain Boomerang, Joel Kinnaman as Rick Flag and series newcomers Idris Elba, David Dastmalchian Flula Borg and John Cena are also joining the film.
The original Suicide Squad, from director David Ayer, was released in August 2016 and grossed $746.8 million worldwide.