This is my favorite note of improv ever because it really encompasses what improv really is. When you improvise you can be anything or anyone, but you have to be believable in order to make it work. Knowing everything moves the scene forward, makes room for more play, and is a built in “yes and” technique.
What some improvisers trip up on is “Well, I don’t really know everything so how can I know everything?” You’re improvising! Make it up fool! Own it!
If you’ve been gifted to play a NASA astronaut, know everything about the cosmos. No, I really don’t know that planet M32 in the 53rd sector of space has an asteroid that’s been circling their earth-like planet, but that sentence does sound pretty damn convincing, doesn’t it? Especially if it’s said as a focused, committed, grounded statement. Own what you say. Say it like you mean it. Don’t apologize for owning your intelligence.
Also, don’t invent. Add to whatever is already happening in the scene organically. If we’re talking about planets, talk about them. You don’t need to veer off and talk about radioactive puppies and be smart about that unless it’s truly, honestly called for.