The Grease spell description reads:
All solid ground in the area is covered withgrease. Each creature standing on the greasy surface
must succeed at a Reflex save or an Acrobatics check
against your spell DC or fall prone. Creatures using
an action to move onto the greasy surface during the
spell’s duration must attempt either a Reflex save or an
Acrobatics check to Balance. A creature that Steps or
Crawls doesn’t have to attempt a check or save.
If I cast Grease on a square where a creature is standing, when does the creature have to make the Reflex save/Acrobatics check or fall prone? As soon as I cast the spell? On the creature's turn? Both?
Furthermore, say a creature starts its turn Prone in a Greased square. It uses an action to Stand. It's now "standing on the greasy surface"-- does it therefore have to make a Reflex save/Acrobatics check or fall prone again?