Spell-casters will always insist that they are able to use their powers during combat melee. The DM must adjudicate the success of such use. The somatic (movement) portions of a spell must be begun and completed without interruption in a clean, smooth motion. The spell as a whole must be continuous and uninterrupted from beginning to end. Once interrupted, for any reason whatsoever, the spell is spoiled and lost (just as if used).
– Gary Gygax, Dungeon Masters Guide (p. 65)
Of course, to adjudicate this statement, the DM must know when the spell starts being cast, when it completes, and which other combatants have opportunity to act in between. What this boils down to is ultimately the initiative system, for which AD&D gives us a fairly complicated system involving the breakdown of a single round into segments and the assignment of a casting time to every spell.
On the other hand, B/X assigns no casting times, and assumes generally a shorter round (10 seconds) than AD&D (1 minute). B/X by the book generally assumes a group initiative and no requirement to declare actions prior to making them happen, thus really there is never an opportunity in B/X for any spell to ever be interrupted.
This is an issue I’ve grappled with in the past. My solution was to assume all spells took a full round to cast, and that any MU that begins casting on his turn does not complete the spell until it is almost his turn again (the beginning of his side’s initiative). I’ve implemented this in our home campaign, and I’m not entirely sure I’m fully satisfied with it. By this system every spell is easily interruptible, or if not at least easy to dodge by simply getting out of the line of site of the caster (who must not move while casting). This makes it harder to cast spells than in AD&D, where there are many spells that take very few segments to complete, and could very well go off well before your opponent has an opportunity to react.
The only other solution I can come up with is to abandon a single initiative roll per combat. Right now I do one roll to determine who gets the first swing and then go around the table from there. By this toke neither side ever gets two actions before the other side. However, if I were to require an initiative roll at the start of each combat round, as is written in B/X, then I could simply require that spell casters declare their intent to cast prior to the initiative roll, and have the spell complete when the caster’s turn actually comes up. Thus, the ability to interrupt the spell would come down to whether or not the other side wins the initiative roll, essentially a 50/50 chance.
Of course, that brings a lot more rolling into combats, and changes the dynamic of combat even when there is no casting at all. We will see more cases where one side gets to make double attacks before the other side gets to react (by losing initiative one round, and then gaining it the next). Is this a bad thing? I’m not sure.
I’d love to hear some other opinions. Please take a moment to cast a vote in this poll, or leave a comment with your own thoughts on how best to adjudicate it.Sorry, there are no polls available at the moment.