Fondest Wish

This illusion spell draws on the target's hopes and desires, showing them whatever they would most like to see at the moment. The effects will generally be to the caster's benefit, but because the spell relies on the wishes of the target, the caster is not in control of what the illusion shows. Indeed, the caster is not even aware of what the target sees until and unless they interact with the target.

The spell produces whatever sensory effects are necessary to provide the illusion. A guard at a lonely outpost sees the caster as another guard coming to relieve him, for example. A patron in a tavern who needs to be distracted sees an old friend across the room. The orc you are fighting sees you fall in combat and turns his attention to someone else. A shopkeeper sees the caster as a wealthy potential patron, or sees the dagger you are trying to sell as a long-sough priceless artifact. 

The specifics of the illusion are thus determined by the DM, and often to continue with a given deception, performance or deception rolls must be made. The guard may see you as a fellow guard come to relieve him, but if you can't bluff him into believing you know the other guards he knows, he is free to become suspicious.