Diggs, who departed the Broadway production of "Hamilton" on Friday, will play an English teacher in the movie. Tremblay is starring as August Pullman, who was born with facial differences that prevented him from going to a mainstream school earlier in life - and then became an unlikely hero when he entered the local fifth grade, reported Deadline.
Roberts plays Tremblay's mother in the adaptation of Raquel J. Palacio's novel and Wilson will portray the father. Lionsgate has dated the movie for April 7.
Producers are Todd Lieberman and David Hoberman through their Mandeville Films. Palacio's book has sold more than 2 million copies since it was published in 2012.
Diggs, 34, originated and played the roles of Thomas Jefferson and Marquis de Lafayette in "Hamilton." He won this year's Tony Award for Best Featured Actor in a Musical.