Kit Young researched cowboys and gunslingers for his role as Jesper Fahey. He trained with guns to the point that the crew found out during pre-production that several trick shots which were planned for CGI were actually stunts Young could do himself.
David J. Peterson, an American language creator known for Game of Thrones (2009)' Valyrian and Dothraki languages, is behind the Grishaverse's fictional languages; he previously helped Leigh Bardugo with the languages in books, as well.
In the books by Leigh Bardugo the adventures of Alina and those of Kaz Brekker and friends, while taking place in the same fictional universe, were not connected, as Kaz and crew were hired to retrieve a scientist who had invented a powerful drug and not the Sun Summoner.
Jessie Mei Li mentioned in an interview that she had a huge crush on Ben Barnes when she was younger, having a huge poster of him on her wall from when he was in the Chronicles of Narnia films.