It's the accomplished actors hired by Warner Brothers for the perfection of their performances. And for more than a decade at this point, Warners had been producing a yearly short films showing them blowing their lines.
It's not as ripe as earlier movies in the series. No more is there a shot of Porky Pig stuttering his way through a bad take, no more competition to see whose vocabulary was foulest -- usually the ladies won those. But there are plenty of examples of well remembered actors in well remembered films breaking down when they can't remember a line of what would become classic dialogue.
