My poll on Spell Interruption pointed almost unanimously to the option of declaring spells first, then rolling initiative. Turns out, though, that this actually exactly as it’s written in B/X. Here are a couple quotes to prove it:
INITIATIVE: To determine initiative, each side rolls 1d6 (the DM rolls for the monsters). The side with the higher roll may move first and attack first in combat for that round. … If combat occurs, the side with the initiative always strikes first in that round. Both sides should roll for initiative each round.
Moldvay, p. B23
That’s not surprising, I always knew it was group initiative. However, it’s kind of funny to note the emphasis on rolling each round — the book even places the word each in italics. To round it out, some fine folks over at Dragon’s Foot pointed this one out to me:
The caster must inform the DM that a spell is being cast and which spell will be cast before the initiative dice are rolled. If the caster loses the initiative and takes damage or fails a saving throw, the spell is interrupted and lost.
Cook and Marsh, p. X11
Yup, it’s right there in the Expert book. Declare spells up front, then roll initiative for each round. Thus each round one side has a 50/50 shot at interrupting spells cast by the other side. Isn’t it funny how most of the issues I struggle with in this game could be easily solved by just sitting down and re-reading the original rule books a little more carefully?