A good or neutral priest can spontaneously cast a 'cure' spell in place of a
prepared spell of the same level or higher. An evil priest, (or a priest of
an evil deity) can spontaneously cast a 'cause' spell in place of a prepared
spell of the same level or higher. The divine energy that the cure or cause
spell substitutes for is converted into the cure or cause spell as if that
spell had been prepared all along.
* Spontaneous casting will displace spells starting from the last spell slot
at a given level. Consequently, if you plan to use spontaneous casting, you
should memorize first those spells you wish to keep in reserve, and memorize
last those spells you would see first displaced by spontaneous casting.
* The tactical implication of spontaneous casting is that priests can
maximize their flexibility by not memorizing any cure spells at all, and
casting any and all that are needed spontaneously.
* Spontaneous casting is available only to Priests and their sub-classes
(druids and clerics); it is not available to bards, rangers, or paladins.
* Spontaneous casting works for all cure and mass cure spells, but not
for heal.