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.
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.