when you know you’re doing it right
Gandalf’s painful encounter with a ceiling beam in Bilbo’s hobbit-hole was not in the script. Ian McKellen banged his forehead against the beam accidentally, not on purpose. But Peter Jackson thought McKellen did a great job “acting through” the mistake, and so kept it in.
I believe, if you look hard, there are more wonders in this universe than you could ever have dreamed of.