Even though the script has gone through careful editing before shooting, it will undergo many changes during the actual filming. Pages in the script are replaced on color-coded paper to help keep everyone on set up-to-date.

The actors' creativity may also come into play in the form of improvisation or as "flubbed" lines that stick - lines that are kept in the final edit.

...not that Elvira has ever missed a line!
at least not a good one!