The way I see it, the illusion can't force them to take any actions or not take any actions. It's not mind affecting. They can know something is wrong and not quite perceive how to pierce the illusion they think might be there. Thats when you resort to the other tactics mentioned. Thats the problem with illusions, people know that sometimes they can fail saving throws, or in character terms be fooled by magic. People usually trust what their eyes see, but they don't always. A non-mind-affecting spell does not have the power to change this.
I have been gone on vacation for a week, and unable to post. In case anyone is still reading I’ll give some more info on the situation.
First however, I want to give an extreme example to show how a PC can believe something is an illusion even though he fails his save. Lets say you have a mischievous gnome illusionist in your party that always summons the same illusionary Ogre to scare off opponents. One day, the gnome gets angry at you and says, “I’m going to summon my Ogre”. He casts the illusion spell. Lets even pretend you have spellcraft so you know what spell its is. The Ogre illusion appears and you fail your save to disbelieved. Must you now fully believe that the Ogre is real and fight/run from it? Must you take illusionary damage when it attacks you?
Here is some more info on our vampire scenario.
We open the coffin and find the illusionary body, then we decapitate/burn it. Upon realizing the Kama is missing, one party member says, “I don’t think this is the real body”. This gives him a will save, which he fails. He then asks the cleric to detect magic. The DM tries to say the cleric would do no such thing since there is no reason for him to believe illusionary magic is at work.
After a little arguing, the player gets to cast detect magic.
The DM does the typical cop-out move of saying, “Everything is magical. Even the walls and floor” the cleric concentrates on the coffin to pick up a particular school of magic, and gets illusion.
Our rogue does a search on the coffin, rolls pretty well too, and finds nothing. We all determine that there must be something there.
If that last body wasn’t the real one then that gaseous form had to go somewhere. That’s when I decide to pour water into the coffin to see if it runs out any cracks or holes. This is where the DM says that our investigations must stop. Our failed will saves and search checks mean that we believe nothing is there and cant investigate further.
One person in our group said he believed that the DMs interpretation of illusions made them to powerful. A low level illusionist could just summon a Pit Fiend and if you failed your will save you were dead. (I don’t think illusions you believe deal illusionary damage, but at least our DM is uses it that way) So he makes the claim that illusionist are now the most powerful class ever, and we should all play one. The DM mentioned that illusions are supposed to be a DM thing anyway.
We had an argument in our last game session about illusions and what it means to fail a will save against them.
We opened a door in this dungeon to reveal a vampire sitting on a throne, next to a stone coffin. We kill the vampire, whose gaseous form travels into the coffin. Upon opening the coffin we find the body and decapitate/burn it.
A failed Will save means the illusion still looks and feels real to you. It doesn’t mean you have to believe it s real. Otherwise it would be mind affecting.
SRD said:A failed saving throw indicates that a character fails to notice something is amiss.
There are two ways to get a saving throw against an illusion. One if from interaction, the other (and the one that applies here) is when a PC says “Something fishy is going on”
SRD said:Creatures encountering an illusion usually do not receive saving throws to recognize it as illusory until they study it carefully or interact with it in some fashion.
So, when a player gets a “hunch” that something is amiss, does a failed Will save make that “hunch” go away? The player is suspicious then the illusion uses a “Jedi mind trick” to say, “These aren’t the druids your looking for”. That’s sounds like mind affecting to me, and illusions aren’t mind affecting. Before a Will save was even called for, we all had suspicions and hunches that something was wrong. Just because we fail to expose the body as an illusion means that all our intuition has to be thrown out the window.
A failed Will save means the illusion still looks and feels real to you. It doesn’t mean you have to believe it s real. Otherwise it would be mind affecting.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.