In an earlier post, I touched on situations that require "pre-declaring" actions in BX. That is to say, before initiative is rolled. This is an interesting artifact of older D&D, as I do not think it is used much in "modern" RPGs. Honestly, we've hardly used it ourselves in my group and I don't even recall using it back in the day. Maybe as my beard grays my gray matter is getting a bit tired.
There are a couple of reasons I think this idea has fallen out of favor. For one, it feels a bit more like a legacy mechanic from D&D's wargame roots than something that was consciously added to the early game; kind of like the whole idea of phases to one's actions (Melee, Ranged, etc.). Newer editions by contrast simply tell the player how much of which kinds of actions they can complete on their turn.
The second reason is that pre-declaring can feel like it is making it harder for players to react tactically in an encounter. If I already declared I was casting Bless, then does that mean I can't switch to Cure Light Wounds when I see our fighter get wounded?
These are valid concerns and they do affect the nature of game play, but before dismissing them out of hand, let's look at some of the benefits of this rule.
- What's good for the goose. Enemy/NPC/Monster spell casters should need to declare as well. Perhaps as a DM you don't want to tell the party exactly which spell is being flung at them, but the PCs should at least be able to see that the pesky goblin shaman is starting to chant and wave his arms around. This lets the characters A) know who the enemy spell casters are, and B) plan accordingly.
- Spell interruption as a tactic. It's true that would royally suck as a low level magic user if your only spell of the day got fizzled because some kobold scored one lousy point of damage from a sling stone. Of course, as mentioned in the first point, that means the PCs can do the same thing to the bad guys (assuming the initiative die is behaving). This can give weaker PCs the opportunity to have a greater effect on the combat, especially if they have a good DEX or other initiative bonuses (like halflings). Also, it gives the aforementioned MU that's tapped out on spells something to do: Just chuck a dagger at any enemy casters and Presto! Old School Counterspell! This also gives players a chance to do something about that incoming Fireball before they're stuck saving for half damage.
- Making magic more than artillery. If your spell casters are vulnerable in this way, it makes the successful use of any spell more useful. When you look at the spell lists in BX, not that many spells are really designed for use in the middle of the fight. Even with a very generous interpretation of what constitutes a "combat" spell (eg healing magic, protection spells, etc.), it only works out to approximately 50%. If a magic-user or cleric's spells aren't as "useful" in a fight, then it gives the players (and the DM) the opportunity to explore some of the other cool magical stuff these characters can do. (For ideas, might I recommend perusing the series of Random Spell Assessments on this very blog!). This also plays into what I've described as a more BX feel to magic, with a more distinctive "flavor" to magic and spell casting.
Personally, I could see going either way with this rule (ignoring or implementing). On the one hand, I know that players generally are loathe to be limited by arbitrary mechanics, but on the other, I do think that this could bring an interesting wrinkle to the game. Some house rules I might consider if I were to try it could include things like: