Extemporaneous describes something that is made up or done without special preparation. It is usually used to describe public speaking.

// Now a seasoned professional, Abby is no longer unnerved when asked to make the occasional extemporaneous speech.

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"The show [Shōgun] lovingly conveys all the details of court society: the Noh performance by torchlight, the extemporaneous verse, the calligraphic stroke falling on a blank sheet of paper." — Ryu Spaeth, Vulture, 23 Apr. 2024

Extemporaneous, which comes from the Latin phrase ex tempore ("on the spur of the moment"), joined the English language sometime in the mid-17th century. The word impromptu, also from a Latin phrase (in promptu, meaning "in readiness") soon followed. In general usage, extemporaneous and impromptu are used interchangeably to describe off-the-cuff remarks or speeches, but this is not the case when they are used in reference to the learned art of public speaking. Teachers of speech will tell you that an extemporaneous speech is one that has been thoroughly prepared and planned but not memorized, whereas an impromptu speech is one for which absolutely no preparations have been made.