Always an Actor

You work as a dustman or in a shoe shop for a while, you move on and everyone forgets about it. You work as an actor in a single role, and everyone refers to you as "an actor" for ever afterward.

Examples:
 * Galaxy Quest is very largely composed of variations of, lampshades hung on, and aversions of this trope.
 * Sunset Boulevard is a tragic variation, making it at least Older Than Radio.
 * The banjo-playing boy in Deliverance is so described in "the other wiki".
 * The various nonentities and Z-listers who attempt to cross over from Big Brother and related shows.
 * Ann Marie in That Girl, far too obviously.