DwD Studios

Like many things, we built a framework and expect a GM to adjudicate, leaning on his preferences and experiences from other games. Here's the way we handle it, your tastes may vary:

If someone believes an illusion, he believes he is affected by it personally - but that won't convey supernatural ability to him.

For instance, if he is stabbed by a hurled illusion spear, he BELIEVES he takes the damage. Keep track of it separately... because if he later becomes convinced of its phantasmal nature the damage can be removed. Some GMs rule the damage is mental and doesn't go away (sometimes mental wounds are far more damaging than physical ones!). If he takes enough believed damage to drop, he drops like normal and might in fact die (all characters who drop in the game get to make a STR check to see if they die or not after the fight's over).

But walking on an illusion bridge won't convey supernatural ability to levitate/fly to the person, and he'll know it's an illusion pretty quickly when he falls.

Also keep note of the number of senses which may be affected. A bridge that affects only sight will fool someone unless he's being smart... but a visual-only illusion of a burning wall that doesn't give off heat (sense of touch) will not fool someone who walks through it that he's on fire.