- During the modernist era, actors rarely saw the complete script of a new play they were to perform. Instead, each actor got the sides for his or her character: that character's lines plus the three or four cue words preceding each line. Even after affordable Samuel French editions of plays became available for all, some old actors would retype their parts in the form of sides, because they felt that learning the role that way made them more alert on stage.
The Historical Dictionary of the American Theater. James Fisher.