Walk on is a term used to describe an actor or actress who has a minor ROLE in a movie or television show. A CAMEO is a brief appearance of a famous person in a movie or television show, usually playing themselves. An EXTRA is an actor or actress who appears in a movie or television show without any lines. A ROLE is a part played by an actor or actress in a movie or television show. TREAD is to walk or step on something. Therefore, the term walk on can be defined as an actor or actress who has a minor ROLE in a movie or television show, appearing briefly with no lines and walking or stepping on something.