The lead role was offered to Chris Hemsworth, but he passed on the role, due to scheduling issues. Then, it came up that the producers invited Gustavo Goulart to perform the lead role (Mitch Rapp), but this rumor was never confirmed.
Dylan O'Brien won the part of Mitch Rapp, beating more established older actors. Being in his mid 20s, the hope is that the actor grows as the envisioned movie franchise progresses. Also, since this movie is based on a prequel novel, it provides the opportunity to start at the beginning of Rapp's career.
"American Assassin" is one of fifteen novels by Vince Flynn set in the world of counterterrorism operative Mitch Rapp. Every book has been a New York Times best-seller, the three most recent entries have been number one New York Times best-sellers, and the series has sold nearly twenty million copies as of October 2015.
Veteran stuntman and martial arts world champion Buster Reeves (Instructor) and Michael Keaton (Stan Hurley) share the Batman mantle. Reeves doubled for Christian Bale and performed Batman's stunt fighting in The Dark Knight trilogy, and Keaton played the Caped Crusader in Batman (1989) and Batman Returns (1992).
The trivia items below may give away important plot points.
The Navy ship near the end of the movie is called the U.S.S. Flynn. It is a fictitious name, as the Navy does not have a vessel with this name. However, it is named after Vince Flynn, the late author of the Mitch Rapp series.
When the last water flushes away from the carrier deck after the explosion, there is a F-35 in the center of the screen. This plane has grey-scale roundels of the Royal Netherlands Air Force on the side of the fuselage. In fact, the RNLAF does have F-35s, they are not equipped to land on or take-off from aircraft carriers. Unless they hoisted the plane on deck, it's impossible for the plane to be there.