How realsitic and responsive are your illusion spells?

[Senko]

Just wondering the various illusion series of spells how responsive are they to realistic conditions? That is lets say you make the illusion of a bouldler and then walk around it with a torch will its shadow move in response to the torch or will it always be in the same spot? If you make a programmed illusion do you have to stipulate everything or could you just go move shadow and have it move depending on where lights fall on the object?

[Mysterious Stranger]

A lot of it will depend on the sub school it belongs to. A phantasm is all in the mind of the target so that should seem very realistic including changing shadows and the like. A shadow is partially real so the shadows would also move. A glamer changes the sensory quality of what it is on so is also partially real. Patterns usually cause mental problems so are usually not designed to deceive people.

Figments create a false sensation by creating an image. That image can block sight so it obviously affects light. Therefore it would create shadows. That is of course assuming the creature looking failed the save. If they make the save it really does not matter because the image is a translucent outline.

[Senko]

Informative so pretty much all of them would shift and change but they'd appear to do so in different way's and have different likelihood of being seen through. Thanks.

[Mysterious Stranger]

The saving throw will determine if they are seen through. So, the likelihood of being seen through is pretty much the same. The same with how they appear. In some cases, like a shadow or glamer the shadow may actually be real. In the other cases it will look real.

When you fool people sometimes their own mind will see more than what is there. Perspective is a strange thing. Ask 5 people about what actually happened at any event and you will get 5 different accounts. Sometimes the accounts will be similar, but often they will be completely different. This is called the Rashomon effect after the Japanese movie.

[Pizza Lord]

Most of the advice is solid. A phantasm is in the target's mind and not actually there, so that's easiest. It looks and reacts however the target would expect it to (in most cases).

As for the others, they don't technically form their own shadows, though the creator typically adds such details if needed. This is a gray area and depends on the GM sometimes, but illusions aren't really there, they can't stop objects, the wind, sunlight, heat or cold. When an illusionist makes an illusory boulder (a figment), for instance, they have to also make the aspects of what would make the boulder believable; cracks, texture, shadows, etc. Obviously there's lots of little details and some of it can be hand-waved with 'it's magic'.

Space Saver:

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In game, an illusion can't react (unless programmed to do so or under the creator's control). An figment illusion of a mirror cannot actually reflect something if the creator didn't have it reflecting it. So a mirror that seems to reflect a room could be done easily enough, since the creator can see the room. How does that apply to differing angles? It would be hard but doable. It could reflect someone walking into the room and looking into it unless it was still controllable by the creator (such as with concentration), who could make the creature's image appear and react like they do (or totally opposite).

The same with an illusion of window on the inside wall of a house. It could look like it shows the outside, maybe even change the time of day out there convincingly and accurately depending on the spell, but it won't make the wall see through, if a person walked past the exterior right where the window should be looking out, they won't appear or be visible to someone inside, whether they failed their save to disbelieve it or not.

Similarly, illusions won't technically stop light or sunlight. The can create an opaque image, but it's only... well... illusory. An illusory pair of sunglasses can dim your vision if you have them on your face. They can make the world you view seem darker or shaded. You might believe your eyes are protected from UV rays, but you could be seriously burning out your retinas looking at the sun. The same for an illusory tent, it might seem shady or dark inside, but the sunlight will still affect you. For most people they probably will wonder why they're overheated and sunburned (even with a thermal element to the illusion that makes it feel cooler inside, it's not real). A vampire could be tricked into walking under what looks like a pavilion or dark tent, and get immolated by the sunlight.
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In the case of the boulder and it maybe not casting a shadow correctly, if a character states that they're looking for such things or mentions something that would be off, that would fall under the inspecting or interacting clause that would call for a disbelieve save. If they fail, maybe it does have a flaw but they didn't notice it or clock the significance of it (or at least, not realize it's an illusion). It could be a magic mirror that just doesn't reflect creatures.

[TxSam88]

Basically illusions are as real as need be to make you believe them until you interact enough for a saving throw. if you fail the saving throw, then it's real enough for you to believe it.

[Mysterious Stranger]

Most of what Pizza Lord is saying under the space saver applies to figments and maybe patterns. Considering figments are the most common type that creates false images it pretty accurate. Shadows being partially real can create shadows.