This is a tricky one. It seems like the formula for when to justify is equal parts instinct and…well, gut intuition.
Justification, for those of you who might not be familiar (Hi Aunt Becky!), is taught as a way to ground scenes that have the potential to leave the audience going “Why are they doing this?”. There are moments on stage when players are running around doing funny voices and throwing improv poop on each other. At least, I have been in scenes like this. And it always feels like “OH GOD, WHY ARE WE DOING THIS? MAKE IT MAKE SENSE, MAKE IT MAKE SENSE”. Which is what the audience is feeling too, times one billion. They want to be in on the joke, in on the world. But there are times when explaining the “why” of a scene takes all the fun out of it, as Mick Napier writes in Improvise:
…a weaker improviser will become uncomfortable with the scene and want to dilute the mystery by answering the question “why are we behaving like this?”
…It would be like Abbott or Costello turning to the audience and saying “In case you didn’t realize it, we are using pronouns in the place of baseball players’ names.”
But, I think, with the help of my fabulous coaches Amos Vernon and Zack Willis, we have stumbled upon a good test of when to justify and when not to justify.