Isabelle Nélisse can't speak English, which is why she's not given much dialogue and speaks more with body language.
Mama's appearance was inspired by a painting by Amedeo Modigliani, owned by Andy Muschietti. The same visual was used again a few years later, as the painting come to life that terrorized young Stan in Muschietti's It: Chapter One (2017).
It would take four hours every day to get Javier Botet into his Mama make-up, and another two hours to remove it.
This is not the first time Javier Botet has played a ghoulish woman, as he previously appeared as the possessed patient zero in REC (2007).
The screenplay is written by siblings Andy Muschietti and Barbara Muschietti, expanded from their previous short film of the same name.