They told me to forget the script because I was going to have to ad-lib everything.
ad lib. abbr.
Latin ad libitum (freely, as desired)
: They danced ad lib until the conductor found his place again
: an ad-lib gag/ a quick ad-lib put-down
A passage or comment, etc, given spontaneously: He forgot his lines and did a stupid ad lib
To speak, play, dance, or otherwise perform a passage not in one's prescribed plan, often with an original and spontaneous effect: The Senator ad lib-bed for ten minutes waiting for the President to show
[fr Latin ad libitum, ''as one wishes'']