Ad hoc and On-the-spot

Ad hoc adjective - Made or done without previous thought or preparation.

On-the-spot and ad hoc are semantically related In some cases you can use "On-the-spot" instead an adjective phrase "Ad hoc".

On-the-spot adjective - At the scene of action.

Ad hoc and on-the-spot are semantically related in extemporaneous topic. Sometimes you can use "Ad hoc" instead an adjective phrase "On-the-spot", if it concerns topics such as unrehearsed, improvised.