Humming centers around the relationship between a sound engineer and a supporting actor working on the post-production of an unfinished film, along with the anecdote of a deceased actor.
| Actor | Role |
|---|---|
 Kim Cheol-yunSung-hyun | Sung-hyun |
 Park Seo-yunMin-young | Min-young |
 Kim Ye-jiNot listed | Not listed |
| SSon Joon-youngNot listed | Not listed |
 Lee Chae-kyungNot listed | Not listed |
 Lee Seung-nohBrother | Brother |