The Dardennes revealed to Variety that they were planning to make this movie with Marion Cotillard as lead, but it didn't work out, so they decided to make Two Days, One Night (2014) with her instead.
The Dardenne brothers re-edited The Unknown Girl (2016) post-Cannes. They had originally planed to make some very minor changes but, after consultation with their editor, decided to make more extensive cuts. They made an additional 32 new edits to the film, which is now 7 minutes shorter than its original Cannes version. The new version was unveiled at the Institute Lumiere in Lyon in June 2016 as part of a retrospective of the Dardenne brothers' films organized by Thierry Frémaux.
Adèle Haenel has been a fan of Jean-Pierre Dardenne and Luc Dardenne for a long time, citing The Promise (1996) and The Son (2002) as her favorite among their works.