Illusions, Disguises, and Such (PF)

[Lord Tyger]

So, I'm running a game currently, and I have a desire to mess with my players via things not being quite what they appear, especially when (hopefully soon) they start dealing with the Fey. As such, I have a range of questions.

First and foremost, what would be necessary/appropriate for a Shadow (the undead) to pose as a shadow (the part of a surface on which light is blocked by falling by an opaque or translucent object, in this case a human shaped object). Would this be a stealth check? A disguise check? A flat perception DC to notice that the person's shadow seems to be moving independently?

Second, in the case of illusions, how often do characters receive saving throws? Is it every time they interact with the illusion, only the first time, or only when they try new methods of interaction?

Third, yes, I know, have a demon with an illusion of a little girl that takes whatever damage he receives. I've seen the suggestion.

[jackattack]

If the Shadow is pretending to be a shadow where there is none, or a larger shadow, or an additional shadow, make it a Disguise check.

If the Shadow is hiding in an existing shadow, make it a Stealth check.

With DEX checks to match sudden or large movement by the target.

-----

Illusion checks:

1. At the beginning of the interaction.

2. When the illusion itself changes significantly.
When the illusory peasant turns into an illusory werewolf, or when the illusory duke suddenly has a dozen illusory arrows sticking out of his back.

3. When a new sense comes into play. (Sensory illusions only, not mental.)
If the illusory guard dog jumps out barking and bites the character on the butt, then just make one roll. But if the character watches the illusory guard dog for a couple of minutes, then listens to it bark as he approaches, and finally pats it on the head, make three saving throws.