Hugo star Moretz will portray the surf-pop icon's daughter Jennifer, while Harry Potter's Grint will play Stan Shapiro, the William Morris Agency mailroom intern who befriended Wilson and became an unlikely confidante.
Aaron Eckhart will lead the cast as Wilson, while Vera Farmiga has signed on to play his lover, Fleetwood Mac star Christine McVie.
The film charts the last six years of Wilson's life.
Director Randall Miller is thrilled with his latest casting news, insisting Moretz is ""the finest actress of her generation,"" and telling WENN she was the first and only choice for the role of Jennifer, who is a producer on the project.
And Miller has high hopes for Grint as Shapiro: ""Rupert's character is the window into Dennis' world. He is the everyman who witnesses the train wreck. Rupert is going to nail it.""
Filming on The Drummer is scheduled to start in June (12) to coincide with the 50th anniversary of The Beach Boys. The film will be shot in California and Georgia.