Overlord's first sequence, which sees the soldiers jumping from a burning plane, was done by rigging a plane on a gimbal, actually blowing up the front, tilting it as if it were actually falling through the air, and sending stuntmen tumbling through real fire.
The movie featured more practical effects rather than the standard cgi effects most movies use. This was done to get a better reaction from the actors involved in the scenes where something gruesome would happen.
Confirmed by J.J. Abrams at the Paramount CinemaCon presentation on April 25, 2018, that the film is, in fact, not an entry in the Cloverfield franchise, despite initial speculations.
The title of the film is, no doubt, in reference to Operation Overlord, the codename for the Allied operation for the Battle of Normandy, which launched the successful invasion of Nazi-occupied Europe. This, alongside Operation Neptune, would become known as D-Day.
In an interview with Jimmy Fallon, Wyatt Russell said he had mono while filming and that he lost around 25 pounds.
One of the filming locations for the film was a decommissioned branch-line of the heritage train line and popular tourist attraction, the Bluebell Railway, in West Sussex, England.
The plaque on the wall of the lab says "Al Pascal Moreau," which is an homage to famous scientists connected with the plot. "Al" most likely refers to Albert Einstein, the famous physicist who encouraged the US to develop the atom bomb during WWII. Blaise Pascal was a famous mathematician and scientist who invented the syringe. And Moreau is a famous fictional scientist from H.G. Wells' novel "The Island of Doctor Moreau," who uses genetic mutation to create horrifying hybrid creatures.
One of the stars, Wyatt Russell, is the son of Kurt Russell, who starred in a similar body horror film, The Thing.