Filming almost took place at the end of 2016, but Robert Downey Jr. wasn't able due to his commitment to the Marvel franchise.
Robert Downey Jr. has been looking at Johnny Depp for an unspecified role
The choice of Arsene Lupin as an antagonist came from the Maurice Leblanc novel where Sherlock Holmes and Lupin had crossed paths.
Robert Downey Jr has been keen to make another Sherlock Holmes film for the best part of a decade as he enjoyed himself immensely making the previous two and found he had a good rapport with co-star Jude Law. However both lead actors had been bogged down since the previous entry with other acting projects and could never seem to find themselves free at the same time, with the Covid-19 pandemic restrictions ruining a hoped for shoot in 2020. The main core cast were finally able to get together for this new film in early 2023 but due to the constant delays up to this point original director Guy Ritchie had to pass on the project due to his busy schedule, handing over the directorial reigns to Dexter Fletcher (who also acted in in Ritchie's breakthrough film 'Lock, Stock and Two Smoking Barrels' (1998)).