Ryan Reynolds appeared in a supporting role in this movie, due to a break in his schedule, after completing principal photography on Deadpool (2016).
Kevin Costner, a director himself, directed the underwater sequence shot in the water tank, when director Ariel Vromen could not, due to scheduling conflicts.
Kevin Costner originally had doubts about appearing in this movie, due to his character, and the film's subject matter, and had originally passed on the project.
During the medical operation sequence, it is not actually Ryan Reynolds, but a doll/dummy made of him out of prosthetic special effects. It would have been more expensive to contract Reynolds for two days work (which is how long it took to shoot the sequence), and also tiresome for him to sit in an operating dentist-like chair.
The nickname of Jan Strook (Michael Pitt) is "The Dutchman". The character is nicknamed such, because the opening scene was supposed to take place in Amsterdam, Netherlands. This was later changed to London, England, when the budget did not allow for filming in Amsterdam, but the writers chose to keep the nickname anyway.