Charging against spears

“A combatant using a weapon with a long reach (spears, for
instance) may choose to attack a closing opponent on the
closing opponent’s number and thus attack simultaneously with
the opponent”

“Spears, pole arms, and certain other piercing weapons deal
double damage when “set” (braced against the ground or
floor) and used against a charging combatant. To set against
charge, the combatant being charged must have equal or better
initiative; this counts as holding an action. Both charging and
charged combatant act on the charging combatant’s initiative
number and are therefore simultaneous.”

So, am I to understand that if you roll better for init, you can choose to hold your action until they charge for double damage, but if you don’t roll better, you can still make a normal attack as they charge?

That’s exactly correct.

Are charges a declared action, or do you just have to sorta hope that you guessed right when you decided to delay your action?

IANAA. I don’t think charge is a declared action. Giving up the initiative advantage is just the risk taken for the payoff of double damage.

That’s what I figured. And I guess it’s generally pretty clear, based on equipment, who wants to be in melee and who doesn’t (though Jutlanders might be tricksy and throw their axes instead of charging with them if they see you’re expecting that)

If you’re carefully tracking distances, you might be able to suss out which is more likely. If they can close with you in a single movement, that means they can move and attack without charging. If they’re farther away, they’d have to charge, but unless they’re wearing heavy armor, being too far to move into melee and attack likely also means being too far to throw axes with their 10/20/30 range.

The NPC’s (or PC’s) personality should affect it too; if they’re doing their best Braveheart imitation, they’re probably a lot more likely to charge at you than your average Roman legionnaire.