I had to look this word up. It is in an article discussing O.Henry'style:
"He lets us see his show from the coulisses."

coulisse
n.
5Fr < couler: see prec.6
1 a grooved timber in which a sluice gate, etc. slides
2 a) any of the side flats of a theater stage b) the space between two such flats

It is clear that the second definition applies. I remember reading that in the remote past, prominent theatre patrons
were seated on the sides of the stage.