If a combatant declares a spell on a round, they cannot move that round – before or after casting.1) If they are struck, use their dexterity modifier to avoid a blow, or move in any other way, they lose the spell for that round. Casters cannot obtain an advantage by losing initiative and thereby “not starting their spell yet” until after they are struck. Spellcasters start trying to cast immediately at the start of the round, any way you dice it (ha!). Chalk it up to slow reaction (initiative), gathering spell components, whatever.
The target of a spell does not need to be declared until the segment the spell completes, though the target must be visible at that time.2)
An attack which hits but causes zero damage doesn't interrupt spellcasting.3)
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