Poll Suggestion: Movies With Only One Major Character

Buried (2010)

@GabrielFox ,

For your consideration:

Silent Running (1972)

In a future where all flora are extinct on Earth, an astronaut is given orders to destroy the last of Earth's plant samples, kept in a greenhouse aboard a spacecraft.

The major character is Freeman Lowell (Bruce Dern), the lone astronaut tending Earth's plant samples.