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…
The ones that come to mind all have other actors in the beginning or end, or voices from other actors. Locke, Buried, 127 Hours, Cast Away, Gravity…
IMDB tells me All is Lost lists only Robert Redford, but I haven’t seen it to be sure.
Wikipedia says All is Lost has only 1 character, but I did find a SPOILER scene that shows Redford by someone else’s arm for only a couple of seconds. I’d figure this is the only scene with anyone else onscreen.
Yeah I was going to mention All Is Lost, it’s just one actor the entire movie. And hardly any dialogue, maybe he says a few words during the movie? I really don’t remember him saying much of anything at all, mostly just grunts/grumbles when things go wrong. Great movie.