Casting Spells (PHB)

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Both wizards and priests use the same rules for casting spells. To cast a spell, the character must first have the spell memorized. If it is not memorized, the spell cannot be cast. The caster must be able to speak (not under the effects of a silence spell or gagged) and have both arms free. (Note that the optional spell component rule [following section] can modify these conditions.) If the spell is targeted on a person, place, or thing, the caster must be able to see the target. It is not enough to cast a fireball 150 feet ahead into the darkness; the caster must be able to see the point of explosion and the intervening distance. Likewise, a magic missile (which always hits its target) cannot be fired into a group of bandits with the instruction to strike the leader; the caster must be able to identify and see the leader.
Once the casting has begun, the character must stand still. Casting cannot be accomplished while riding a roughly moving beast or a vehicle, unless special efforts are made to stabilize and protect the caster. Thus, a spell cannot be cast from the back of a galloping horse under any conditions, nor can a wizard or priest cast a spell on the deck of a ship during a storm. However, if the caster were below decks, protected from the wind and surging waves, he could cast a spell. While it is not normally possible to cast a spell from a moving chariot, a character who was steadied and supported by others could do so. Your DM will have to make a ruling in these types of extraordinary conditions.
During the round in which the spell is cast, the caster cannot move to dodge attacks. Therefore, no AC benefit from Dexterity is gained by spellcasters while casting spells. Furthermore, if the spellcaster is struck by a weapon or fails to make a saving throw before the spell is cast, the caster's concentration is disrupted. The spell is lost in a fizzle of useless energy and is wiped clean from the memory of the caster until it can be rememorized. Spellcasters are well advised not to stand at the front of any battle, at least if they want to be able to cast any spells!
==Spell Components (Optional Rule)==
When your character casts a spell, it is assumed that he is doing something to activate that spell. He may utter a few words, wave his hand around a couple of times, wiggle his toes, swallow a live spider, etc. But, under the standard rules, you don't have to know exactly what he does to activate the spell. Some of this can be answered if your DM uses the rules for spell components.
The actions required to cast a spell are divided into three groups: verbal, somatic (gestures), and material. Each spell description (found in Appendices 3 and 4) lists what combination of these components is needed to cast a spell. Verbal components require the caster to speak clearly (not be silenced in any way); somatic components require free gestures (thus, the caster cannot be bound or held); material components must be tossed, dropped, burned, eaten, broken, or whatever for the spell to work. While there is no specific description of the words and gestures that must be performed, the material components are listed in the spell descriptions. Some of these are common and easy to obtain. Others represent items of great value or scarcity. Whatever the component, it is automatically destroyed or lost when the spell is cast, unless the spell description specifically notes otherwise.
If the spell components optional rule is used in your campaign, your wizard or priest must have these items to cast the spell. Without them, he is helpless, even if the spell is memorized. For simplicity of play, it is best to assume that any spellcaster with any sense has a supply of the common items he is likely to need—wax, feathers, paint, sand, sticks, and fluff, for example. For expensive and rare items, it is perfectly proper for your DM to insist that special efforts be made to obtain these items. After all, you simply cannot assume your character has a valuable pearl handy whenever he needs one!
The three different aspects of spell components also change the conditions under which your character can cast his spells. No longer does he need to be able to speak, move, and use some item. He only needs to fulfill the required components. Thus, a spell with only a verbal component could be used by a naked, bound spellcaster. One requiring only gestures could be cast even within the radius of a silence spell. Most spells require a combination of components, but clever spellcasters often create new spells that need only a word or a gesture, enabling them to take their enemies by surprise.