Rebel sentries were members of the Rebel Alliance whose duty it was to keep an eye on arriving and departing starships. They performed this task from atop a watchtower situated close to the Great Temple, which acted as the Rebellion's main base of operations on Yavin 4.
The initial appearance of the Rebel sentry was in the original Star Wars movie, Star Wars: Episode IV A New Hope, which premiered on May 25, 1977. Lorne Peterson from Industrial Light & Magic portrayed the sentry. He mentioned that he took on the risky role of standing in the watchtower because he was the only member of the special effects team without children. The prop Peterson held in the scene consisted of two light meters that were taped together.
The Rebel sentry was seen again in Rogue One: A Star Wars Story, the first anthology film in the Star Wars series, which came out on December 16, 2016. John Swartz, who also co-produced the movie, played the part.