The PCs this week tried to do something that stumped me, and I am sure this question has been asked before. Caught between a rock and a hard place, the PCs decided that they would cast an illusion spell to try to catch the enemy off-guard. Thus far, everything made sense.
However, other questions began to be raised: illusion spells are generally "will disbelief, if interacted with," so presumably the enemy wouldn't be given a will save until they interacted with it. As well, the PCs started to ask if they could use it to flank, and generally other things that would happen if the enemy believed the illusion (would it provide cover? Threaten squares? What happens if it is attacked?).
How do illusions work, game-mechanically, when someone believes an illusion? I added the Pathfinder, 3.5, and 3e tags as I assume the same (or similar) ruling would apply to all three.
Considering the illusion school covers a variety of spells, I am primarily concerned with figments; other sub-schools seem better defined. E.g. silent image, minor image, major image and the like.