Spellcasting

Movement

If a combatant declares a spell on a round, they cannot move that round – before or after casting.1) Good post here: https://www.dragonsfoot.org/forums/viewtopic.php?p=140735#p140735 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.

Targeting

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) Originally, we required targets to be declared during the declaration phase. If the target died, disappeared, or moved out of an AoE, the spellcaster was out of luck. However, much later, we realized that the rules do not appear to say this, and since no one could remember where we got it from, we removed that requirement. If you know of a rule we missed, please let us know.

Interruption

An attack which hits but causes zero damage doesn't interrupt spellcasting.3) PHB 45, “Chant”, “An interruption, however, such as an attack which succeeds and causes damage, grappling the chanter, or a magical silence, will break the spell.”