* When the comedian forgot his lines, he had to ad-lib for a few minutes. (meaning he improvised)
* The actor's ad-libbing was so funny that the audience burst into laughter. (meaning he improvised)
* The band decided to ad-lib a new bridge into their song during the concert. (meaning they improvised a new part)
* The interviewer's ad-lib question caught the celebrity off guard. (meaning the question was improvised)
* It's always fun to watch actors ad-lib during a play. (meaning they improvise)
You can also use "ad-lib" as a noun:
* The comedian's ad-libs were hilarious. (meaning his improvisations were funny)
* The singer added a few ad-libs to her performance. (meaning she added improvisations)