When you have kids you want to be able to go to movies and take the family too, and actually all enjoy it together. I don't think there are that many great, live action family movies that everybody can enjoy.
I think you never want to have to go into the scene having to improvise; you want to make sure its working on the page. But I do like to have the ability to try stuff just in the moment, to give it some sort of spontaneity.