Extras/other people in the background are acceptable to meet the criteria but ideally with no human/entity on the screen at all that isn’t played by the same actor.
Movies like ‘Men’, ‘Moon’ or ‘The Nutty Professor’ don’t meet this criteria for example, due to the exceptions of characters played by other actors.
And it has to be somewhat mainstream and not a low budget student film or something.
Edit: I also meant that they play multiple characters…
Buried, Ryan Reynolds is the only one on screen (not playing multiple characters, just one), though there are voiced characters from other actors on the phone.
I always forget about that movie, but it’s really good.