The stage manager is, basically, God, in the play Our Town by Thornton Wilder. For those who have never had the chance to see a production of Our Town, the stage manager is the most prominent character on stage. The stage manager is omniscient, omnipotent, and can change the place in time, at any time. The stage manager knows the past, knows the present, and knows the future.
The stage manager already knows all the characters' fates at the beginning of the show. The stage manager can fast forward the day, say, three hours at any time. The stage manager can take the play back in time. For example, in the third act, the stage manager takes the now-deceased Emily Webb back about 15 years to her eleventh birthday, in a "ghost of Christmas past"-esque style.
The stage manager also "acts" the role of the minister who marries George Gibbs and Emily Webb at the end of the second act.
That is, the stage manager does not change the costume, but the stage manager just does his minister act. Mr. Foo does not play the stage manager and the minister. The stage manager plays the minister.
Basically, the Stage Manager is God.