To throw yourself in the path of an attack directed toward another character, make an attack roll. If this intercept roll hits an armor class as good as the attack roll being intercepted, the interceptor becomes the new target of the attack and moves between the attacker and the original target. The decision to intercept must be made prior to the attack roll.
Fighters may perform one intercept reaction per combat turn. Characters of other classes may only perform an intercept if they hold their action. Retainers directed to intercept attacks may be required to pass a morale check.
I want to add a sentence about how intercepts can only be attempted if they make sense logically, or are supported by the fiction (or whatever), but don’t have quite the proper language down yet.
An interceptor must have been targetable by the attacker and within melee combat range of the original target?
Avoids being excessively concrete, but I think covers the cases you’re trying to address.
I’m not sure I understand “hits an armor class as good as the attack roll being intercepted” – do you mean “equals or exceeds the attack roll”?
“An armor class as good” is meant to be suitably general so that it can apply to games with both ascending and descending AC. With ascending, you can just say equals or exceeds the attack roll (because both sides are rolling d20 and adding bonuses), but with descending THAC0 tables allow some creatures or characters to hit better ACs with lower rolls, so you can’t just say higher rolls are better in the case of comparisons.
I say you go full on vague / OD&D: “If a player is in a suitable position to intercept an attack directed at one of their alies they my attempt to do so. …” And then the DM / Player can decide what ‘suitable position’ means.