Illusions have a special kind of resistance. When a person or creature overcomes the effect of an illusion, it does not Misfire. A Misfire can only occur when the illusion is first cast, but creatures can "see through" the illusion later.
When a PC is first confronted with an illusion, they must make a Saving Throw to see through the illusion. The Saving Throw uses an ability based upon the kind of illusion they see, and this requires GM judgement. The illusion of a collapsing ceiling might use Constitution, but whenever in doubt about which ability to use the default is Intelligence.
If another character tries to assist a PC in seeing through an illusion the PC may add a +4 to their Saving Throw, but only if this is communicated effectively.
Often players will deduce when something in the game world is an illusion, even when their characters fail their Saving Throws. Remind yourself, and them, that this is an ideal time to role-play as if the illusion were real.
NPCs who view illusions only see through them if their Hit Dice exceeds the Spell Power of the illusion. Another character could try to get an NPC to see through an illusion. If this happens, add the character's Hit Dice to the NPC's Hit Dice. If the combined total doesn't equal or exceed the Spell Power then the NPC can't see through the illusion and may believe the character is affected by their own magical illusion, delusion, or mental illness.
Illusions that "kill" characters put them into a trance-like coma that only ends when the spell ends, or the magic affect on their body and mind is dispelled. Illusionary attacks and affects always use the creating wizard's Abilities and Skills to determine their effectiveness. Thus, a wizard who has created an illusionary minotaur to fight a group of goblins uses his own Attack and Armor scores when determining how well the illusion fights.
Un-dead are immune to all illusions and their effects. Some creatures, especially Demons, have natural resistances as well. In general, illusions with Power high enough to affect them will modify the behavior of NPCs and monsters.
I don't know if I'm taking the "bite" out of illusions by ruling it this way. Illusions were definitely more prominent in 1st edition AD&D, but I think they required a lot more arbitration on the GM's part. I might have to write guidelines for how illusions work, but for now the comparing of Hit Dice will have to do.