Debate can enhance any decision that requires NPC cooperation.

A diplomacy check reflect the persuasive abilty of the characters involved, and a success means a disposition to do what one can to help as described under the headings friendly and helpful. But what is possible? Even helpful characters won't do what is impossible. So if your plan has elements that can be thought as not practically possible even helpful allied might not agree to the aid you have asked for.

Debate is based on a knowledge skill you have. In theory you could debate with any skill, but a debate over the best application of the Hide skill is probabaly not a good use of time, if in fact the Hide skill is called for. Some skills like Warcraft, Administration, or Appraise could be subjects of debate. Winning a debate does not persuade people to do things that your Diplomacy check was unable to get them to do. The idea of having a debate is to add depth to a diplomacy encounter by having skeptics challenge your plan without having to be hostile. After all, friends don't always agree on the best course of action.

Suppose Adelstan the Just, noble Rjurik, is sent by his king to the friendly court in Hogunmark to win their cooperation in an upcomming war. Due to game history, Queen Freila is helpful to both the realm of your character and your PC, Adelstan, as well. Making the Diplomacy check is not a problem. The Queen and the court want to help and share you desire for victory over this foe. However, suppose Hruthwulf, the High Steward, objects that at the given time of year an army of Hoguns cannot cross the Hjarring river as you propose.

Hruthwulf has stated a thesis and the DM selects a knowledge skill that reflects the skill most associated with the thesis. If a non-knowledge skill is more appropriate then use that skill, but for similar skills, a knowledge skill is better suited for debate. The DM might select either Knowledge (Nature) or (Geography) based in whichever Hruthwulf has more of. People argue from what they know best (if they are wise).

Hruthwulf is a 6th level ranger (or more likely a Noble 1/Ranger 5, but let's put that aside). He has 12 ranks of Knowledge (Nature), so the DM selects that skill as the issue being debated. Hruthwulf has 12 knowledge points, as many as he has ranks in the skill. Your PC, Adelstan, has 5 ranks of Knoweldge (Nature) and you realize you have a problem. Your character has 5 knowledge points. As you debate, the character who loses an exchange loses knowledge points.

An exchange is basically an opposed skill check based on the skill being debated.

Hruthwulf as 12 ranks, a +2 Int bonus, and a +2 synergy bonus from his many Survival skill ranks, for a +16 bonus made to his objections and replies.

Adelstan has 5 ranks, a feat that allows him to use his Charisma bonus whenever addressing groups, so applies a +3 bonus, also gets a +2 synergy bonus for Survival ranks, and so has a +10 bonus for objections and replies.

If you want a long drawn out debate, each knowledge point can be contested individually. Or you can assume that each exchange resolves 1d4 knowledge points. If this is the case, Adelstan will probabaly be able to lose only two exchanges before he is revealed to have proposed a plan that is simply impossible because of the depth and speed of a major river. It would probabaly take six or so failed exchanges for Hruthwulf to be silenced on this matter.

If Adelstan loses this debate, and he probabaly will (because no one anticipated Hruthwulf's opposition), Hogunmark still wants to join your PC's realm in attack, but a new plan must be proposed, either by the PC, the Hogun court, or by word from home. Perhaps Thorjak promices to quiet the river so the PC's plan can proceed. Perhaps the Hoguns propose collecting many boats to cross the river and delay the attack by a month (as well as allowing enemy scouts to notice a large collection of boats and anticipate a river crossing). Perhaps they just offer you a few GB to purchase mercenaries on the other side of the river.

If you have already seen my description of Harald Khorien, you might have noticed that he had memorized two books as feats, and that these books gave him bonuses in disputations.

Normally on a subject like Knowledge (Natural Philosophy) - what we might call science today - Harald Khorien has 13 ranks of Knowledge (Natural Philosophy) and so has 13 knowledge points in such a debate. His +4 Int modifier applies, and having memorized Questiones Naturales, he gets a +4 bonus to K(NP) during disputations (debates). He also has Knowledge Focus for this skill for an additional +3 bonus. So his bonus for Objections and Replies is +24. Having memorized Timeaus he recives an additional 10 knowledge points on subjects of natural philosophy, so he now has 23 knowledge points. Harald Khorien is a formidable debater of natural philosophy indeed.

You can attempt to use Bluff in arguments. There are two kinds. You can bluff on the facts, but these must refer to conditions that cannot be verified in the debate. You can claim to have a catch of boats assembled secretly in Jankaping. Of course when the Hoguns show up expecting boats, you will still need to get them across the river. Second, you can make an ad hominem attack. You could suggest that Hruthwulf was a coward or traitorous. This would be a Bluff as well, although doing such a thing would breech any friendship with Hruthwulf and might result in his excercising his rights to physical combat to settle this calumny. Generally using a Bluff in a debate is a dishonorable act and will generally have consequences now or later. Scoundrels might hope to bluff now and fill in the gaps later, but that's obviously a risk.