Andrew Garfield was originally cast as the Monster, but had to drop out due to scheduling conflicts, and was replaced by Jacob Elordi. The makeup team spent nine months crafting Garfield's look as the Monster, but had only nine weeks to rework it for Elordi in time for filming.
In a previous reincarnation of this production, it was planned as part of a new series of remakes of the traditional Universal Studios monster universe, which included Dracula Untold (2014) and La Momie (2017). Eventually, Dracula Untold (2014), for some reason, was left out of what was called the Dark Universe, which officially started with La Momie (2017) and was going to include remakes of Dr. Jekyll and Mr. Hyde, Creature of the Black Lagoon, the Wolfman, the Invisible Man, and Frankenstein. However, due to Dracula Untold (2014) and La Momie (2017)'s bad box office performances, the idea of a connected cinematic universe was scrapped. As a result, this film became the intended standalone film, as always planned by director Guillermo del Toro.
As early as the late 1990s to early 2000s, actor Doug Jones was attached to play the monster. Jones has worked with director Guillermo del Toro in many films, including Hellboy (2004), Le Labyrinthe de Pan (2006), Hellboy II : Les Légions d'or maudites (2008), and La Forme de l'eau (2017), respectively.