New Latin ad libitum
Ad-lib is used to describe individual moments during live theatre when an actor speaks through their character using words not found in the play's text. When the entire performance is predicated on spontaneous creation, the process is called improvisational theatre.
In film the term ad-lib usually refers to the interpolation of unscripted material in an otherwise scripted performance. In interviews, Dustin Hoffman says he ad-libbed the now famous line, "I'm walking here! I'm walking here!" as "Ratso" Rizzo in Midnight Cowboy.
Live performers such as television talk-show hosts sometimes enhance their reputation for wit by the delivery of material that sounds ad-libbed but is actually scripted, and may employ ad-lib writers to prepare such material. Some actors are also known for their ability or tendency to ad-lib, such as Peter Falk (of the series Columbo), who would ad-lib such mannerisms as absent-mindedness while in character.