A question, and an answer

Why would all these parallel worlds have a similar dinner party, if each character, in each world, will have slightly different life paths? Surely the likelihood of having such a dinner party on such a day is highly dependent on a life long chain of specific events?

The answer is yes, this is true. BUT there are infinite worlds here, and only those worlds that, by chance, had gathered a dinner party of these individuals would mass-hysterically produce behavior like what we witness. Only those worlds, with those dinner parties, would force the cross worlds activity.

In countless other worlds, we would just have the house owners sitting and sipping wine in the darkness, waiting for the power to come back. In other words, given infinity, we are bound to see those - and only those - worlds in which they gather and raffle each other up as they did here..... and besides, we only see a handful of even those worlds.

PS: The mixing starts when the phone brakes at the very beginning in the car. The team we meet are all from different worlds from the very get-go (or most of them). Evident in not remembering or misremembering each other's backgrounds, as we learn through their small talks - like if the Buffy guy ever acted in Roswell or not.

Amazing flick.