walk-on

Short definition from Wiktionary

Noun

1. 1
   A student athlete who wants to try out for a college sports or 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.

Verb

1. 1
   To come onstage.
2. 2
   To join a college sports team without being recruited for it.