Have you ever been in the audience of an improv show, and watched something that, for lack of a better word, seemed magical? Where the cast seemed to act as a unit, moving in a syrup-like cohesion of awesomeness. That’s group mind, and it’s pretty freaking awesome.
Group mind (definitely different but also kind of similar to group think but without all of the negative connotation of cults and stuff like that) is the idea that there exists a moment in time when all of the actors on the stage are so in tune with each other that creating takes no effort at all. It’s like all of their brains have fused into one, and comedic gold comes pouring out.
Why does it happen? Several reasons. One may be that all of the actors are so acutely present on that stage that they can recognize every facial tick, inhalation and gesture and know what is about to happen next, and act accordingly. But they aren’t thinking, processing or analyzing. They are simply acting. It’s when a performer’s rational brain is turned off that they do their best work. Another reason is that those actors know each other innately. This occurs from time spent outside of the rehearsal room, grabbing a bite to eat and learning about what makes the other tick, how their brain works. Put all of that together and you get group mind, or, as the audience sees it, magic.