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That's a really good question. I've seen movies you might consider "art films" that have just one character for most of or all of their duration and they turn out fine. But they also might not appeal to the general movie-goer. Some mainstream movies pull it off for certain durations, like Tom Hanks in Castaway, or they allow the singular character to communicate with off-screen people like in Buried.
I really think if you make what's happening on screen interesting, it doesn't matter that there aren't multiple characters speaking to each other. But if that's too risky, I think I'd settle for a Metroid that has a few invented characters, or where Samus sometimes communicates with the Federation during her mission.