Hey, It's That Guy!: Chilled Chaos, a relatively well-known Let's Player (who also played through the original Stanley Parable mod), helped out with one of the trailers (this one) by essentially doing his usual shtick while playing through some of the game.
What Could Have Been: The Museum ending, aside from showing off a lot of the game's assets, demonstrates several unused and unimplemented ideas, including further Ask The Narrator trailers (in the vein of the Raphael trailer) and a battleground ending.
A planned ending, where the game turns into a sci-fi First-Person Shooter game with its own narrator who ends up in conflict with the Narrator, was never implemented, but shows up in the Museum.
According to Word of God in this Reddit AMA, one ending was originally intended to have the Narrator forcing Stanley into repeatedly jumping off a ledge until he dies. This was scrapped for being too brutal, and replaced with the ending where Stanley commits suicide by the same method against the Narrator's orders.