How would you handle an illusion someone saves against then interacts wtih?

[Senko]

I was thinking about illusions and got to thinking what happens to someone who interacts with something they save against. That is say you cast the illusion of a boulder blocking a path and a person saves and walks through it. What would they see when in the middle of the illusion? It wouldn't be darkness but would they just see things as normal around them, would they see a translucent illusion of the boulder surrounding themselves as they stand in the middle of it?

[Mysterious Stranger]

A successful saving throw against an illusion reveals it to be false, but a figment or phantasm remains as a translucent outline.

Assuming it is figment or phantasm it would not create any darkness. Patterns are all in the mind so it would also not create any darkness. So, most illusions are not going to create any darkness or really impact the character that saves. Glamers only change the sensory qualities so cannot make something appear out of nothing. Shadows are the only questionable illusion.

Shadow: A shadow spell creates something that is partially real from extradimensional energy. Such illusions can have real effects. Damage dealt by a shadow illusion is real.

Shadow spells usually have a percentage of being real. If the roll for the effect is under that percentage, it is treated as real. That would mean the character is not walking through the bolder even if they make the save. If the roll is over the percentage, I would treat it similar to a figment or phantasm.

[Senko]

So . . . that's a they'd see the translucent boulder still?

[Mysterious Stranger]

Most of the time that is what they would see. With a Phantasm they might not even see that. Shadows are the questionable ones. I would have the show the transparent outline just to keep it simple.

[Bellona]

Some illusion spells are special corner cases. IIRC, Illusionary Wall is one of them.

[Azothath]

A GM can describe the effects of succeeding vs an illusion in various ways. The creature can still interact with it (such as wanting to try to fool(aka Bluff) an observing spellcaster).
It is in the same ballpark as "spellcasting manifrustrations".

Exactly what the illusion is of is a matter of the spell description, player creativity, and GM acceptance. So lighting levels(SU Dark, dark, dim, normal, bright, SU bright), emptiness(void), fogs/smoke, etc depend on the spell description.

[Mysterious Stranger]

The first line of my first post was copied and pasted from the core rule book. If a specific spell says that it works different that is a case of the specific taking precedence over the general, but otherwise if the illusion is a figment or phantasm it shows up as a translucent outline. A GM can decide that illusions work differently in his games, but that would be a house rule.