According to Variety, Roberts, 39, has joined the cast of the family drama which also stars Emily Watson and Carrie-Anne Moss. The project will be produced by Senator Entertainment.
Filmmaker Dennis Lee is adapting his own screenplay for the film, which will be his first feature film.
Fireflies in the Garden is the semi-autobiographical story of Lee, and follows a family’s love and commitment when a tragedy strikes.
Filming is set to begin in Texas in March.