walk-on

noun
  1. 1

    A student athlete that wants to try out for a college sports/athletic team without the benefit of a scholarship or having been recruited.

  2. 2

    An actor of a small (or "bit") part in a theatrical production or film, often without speaking lines.

  3. 3

    Such a part in a play or film.

  4. 4

    One who arrives and boards transport (such as a train or ferry) without having booked in advance.

Source: https://en.wiktionary.org/wiki/walk-on