To cast a spell you spend 1 standard action.
MAGIC WORDS
To cast a spell, you have to be able to speak
in a firm voice, as loud as a normal conversation (no whispering).
MAGIC GESTURES
To cast a spell, you have to have one
hand free (not holding anything and with no shield strapped to
that arm) so you can make magical gestures and point at the
target of your spell.
HOLY SYMBOL
If you’re a cleric, you have to wear or carry
your holy symbol to cast your spells (you don’t have to hold it).
CONCENTRATION
You have to be able to concentrate
to cast a spell. If you are adjacent to an enemy, that enemy
distracts you enough that you can’t cast a spell. The exception
is a spell that has you make a melee touch attack—you are able
to cast that spell next to an enemy.
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 Source: Paizo Blog, Paizo Publishing LLC |
TOUCH SPELLS
If a spell has a range of “touch,” you
cast it and then touch the target as a free action that round.
Casting it on yourself or on an ally doesn’t require an attack
roll. If you want to cast the spell on an enemy, you have to
make a melee touch attack. If you don’t touch someone
with a melee touch attack spell in the round when you cast
it, you can try again later as a standard action. This is called
holding the charge. If you cast another spell while holding
the charge, you lose the original spell. You can cancel the held
charge by spending a standard action. | 
