Expect table variance but... the issue isn't the prisoner's Will save. It's that the spell doesn't do what you're trying to do.
The charm person spell simply makes its target view your actions in a positive light, and treats you as an ally. That doesn't necessarily prevent them from lying or keeping secrets from you. There are people in your life that you likely love dearly but don't tell all your personal details or plans to.
The idea behind the charm spells is that what you do is trusted. It doesn't enslave or force the target to do things for you.
The zone of truth spell is second level and that one is designed to prevent lying.
I understand the idea here is "hey, I'm your friend... let me in on your plan to assassinate the king." But this spell is more about "hey, I'm going to go ahead and bring this dagger with me in while I see the king, but it's cool." If I were planning on doing something secret, I wouldn't be sharing that even with people I generally trust.
Also, don't get too wrapped up in the word "ally". The game uses that word in a particular sense. In this case, an ally won't attack you, and may be used to produce flanking. It's not really meant as "are part of the inner circle of the Illuminati too."
Knock the person to -1 hp or so [possibly via non-lethal damage to lessen the risk of a real accidental kill].
This renders them unconscious [unless an orc or a Raging Vitality barbarian].
Unconscious creatures are automatically considered willing [though they do still get their SR if any].
Now they automatically fail their Will save to the Charm Person that is cast.
Heal them to +0 hp or more.
Beat them on an opposed Charisma roll.
Win.
Urk.
While rules-as-written you're correct, it's generally considered kind of... um... creepy and um... creepy to use it that way. If you're trying to dimension door an unconscious ally away from a battle to heal up, that's okay. But there's a whole context about doing bad things to someone unconscious and considering them willing that's over the line for a lot of people.
"Charm Person: The main thing to remember about charm magic is that it is not a compulsion (that is a different subschool of enchantment), which means it doesn’t directly force someone to do something. Instead, the spell basically makes someone feel like the caster is a friend, and puts what the caster says in the best possible light. (...) [B]eing someone’s friend doesn’t mean the caster gets to dictate everything they do, and even the opposed Charisma check the spell grants can only go so far; it doesn’t compel them to act exactly as the caster desires." UI pg. 157
So it can indeed help for an interrogation, but only for things that the target would tell their friends. And you better treat the target well - I know I wouldn't tell my friends jack if they currently had me bound to a chair.
The zone of truth spell is second level and that one is designed to prevent lying.
Zone of Truth is utterly horrible, because you don't know if the a creature is actually affected or not, whereas the creature is either unaffected, or knows it's affected. The "Sense Enchantment" use of Sense Motive may help, but that's a DC 25 check. If you can reliably make that, you can probably catch lies anyway.
Abadar's Truthtelling is a 1st level spell, and you get a visual confirmation if the target is affected (plus, the caster would automatically know anyway, as is the case for all single-target-spells).
As has been said, it isn't "I cast it and they spill their guts" spell, but if used cleverly it could be used for this purpose. It would make diplomacy easier.
Several conditions give penalties to will saves, but they could be seen as attacks and then he could get the +5 bonus for "being threatened or attacked by you or your allies". On alcohol; "... In general, a character can consume a number of alcoholic beverages equal to 1 plus double his Constitution modifier. Drinks consumed in excess of this total cause the character to become sickened for 1 hour per drink above this maximum. ..."
"Sickened: The character takes a –2 penalty on all attack rolls, weapon damage rolls, saving throws, skill checks, and ability checks."
So yes, you do get a -2 to saves if you drink enough, though forcing him to drink could probably be considered an attack.
Getting the character drunk can help, but only if done voluntarily. As Valandil points out forcing alcohol down a persons throat would be considered an attack and as such give them a +5 bonus to their save. But if you can get them to willingly drink enough to get them drunk it will also impose a -2 penalty on their skills including bluff and sense motive. Even without charm person this is going to help get the information you need.