Plex doesn’t use any side-car files for metadata or actor photos.
Set the Cast List Source to TheMovieDatabase
in the properties of the Plex Movie agent

Additionally, activate Plex Movie as a secondary agent under
Settings - Server - Agents - Movies - TheMoviedatabase
If your files are in the mp4/m4v format, make sure they don’t already contain actor infos. Because these embedded actor lists come always without photos.
If you want to tell Plex to disregard embedded metatags, you need to change the default order of metadata agents:
Go to
- Settings - Server - Agents - Shows - TheTVDB
In there, grab the line ‘Local Media Assets’ with your mouse and drag it downwards, so it ends up being at the bottom of the stack of active agents.
Repeat the same under
- Settings - Server - Agents - Shows - TheMovieDatabase
- Settings - Server - Agents - Movies - Plex Movie
- Settings - Server - Agents - Movies - TheMovieDatabase
Afterwards, clear the local agent cache https://support.plex.tv/articles/202967376-clearing-plugin-channel-agent-http-caches/
and Refresh Metadata on the affected movies.
Only movies, which have actors with photo listed on themoviedb.org will ever get actor portrait photos in Plex.