To give the appearance of seamlessness to the space ship, the crack around the door was filled with putty, then painted over. When the door opened the putty was torn apart, making the door seem to simply appear.
To increase the sense of reality, some of the most famous broadcast journalists of the time were hired to do cameos as themselves. These included Gabriel Heatter, H.V. Kaltenborn and Drew Pearson.
Whether the makers of the movie intended or not, there is a striking resemblance between Klaatu and the head of the Manhattan Project, Robert Oppenheimer. Oppenheimer was known to make corrections on the blackboards of theoreticians at the Project, similar to the scene where Klaatu corrects the work of Professor Barnhardt. Oppenheimer's other-worldly brilliance and association with destructive power that could threaten the existence of the world seem like more than a coincidence.
Writer Edmund H. North was a former army officer who wrote the script in response to the proliferation of nuclear weapons during the Cold War.