Full disclosure: most of the films on this list of movies with only one actor actually have other actors in them. However, the bulk of the narrative and the heft of the conflict is placed upon a single protagonist who is all alone. So "films with just one actor" speaks more to the plot than the actual number of actors in the movie. Even films like Cast Away have more than one actor, even though the story is almost solely about Chuck's survival. Make sense?
Survival is a common theme among movies that only have one actor. What lengths will Aron (James Franco) go through to survive being trapped by a boulder deep within a canyon in 127 Hours? What will Dr. Neville (Will Smith) learn from the zombie-like monsters in I Am Legend in order to find a cure and save humanity? Will Robert Redford’s character in All Is Lost manage to survive being lost at sea all alone?
Of course, survival is not the only theme in movies where there's only one actor. There are other plot lines. However, no matter the narrative, one thing is for certain: it takes a skilled actor to solely command an audience’s attention for the entire duration of a movie.