The knight that appears in young Vivienne's visions carries a familiar sword: Anduril, Aragorn's sword from The Lord of the Rings: The Return of the King (2003), which was gifted to Viggo Mortensen by Peter Jackson at the end of filming.
Viggo Mortensen did not intend to act in the film. "Late in the game", the actor who had originally been cast as Holger left to work on a different project. Vicky Krieps suggested he take the role himself.
On casting Vicky Krieps, Viggo Mortensen said, "I thought she would be great as this character. Very quiet but with a particular point of view. I needed someone I could believe was from the 19th century, and I don't believe that about all actresses I see for various reasons."