Inception is a notoriously confusing movie with an astoundingly complex world. To make itself understandable, then, it requires someone that TV Tropes calls The Watson: A character (like Sherlock Holmes’ John Watson*) who is as confused as we are, and who asks all the questions we would ask, so that the movie can explain itself to us. (Hello, Mr. Exposition.)
In Inception that character is of course Ariadne, played by Ellen Page. She’s the audience surrogate who pipes up with questions like “Who or what is Mr. Charles?” and “Whose subconscious are we going into again?” Or, “What?”
*Correction, July 1, 2013: This article originally misidentified the Sherlock Holmes character John Watson as James Watson.