Perception - Disbelief

Sometimes a character will perceive something that is not real. This might be the result of a magical illusion, a drug or anything else capable of tricking the character's perceptions. Illusions and hallucinations may take one of following two forms, if not a mix of both.
The first one is the character perceiving something that is not there. This could be an illusory wall or pit, a sound, a smell or anything else that a character's sense could perceive if it was real.
The second one is masking or altering something that is actually there. This could make a door seem like a wall, or turn a character completely invisible, or mask the sound of an ongoing battle.
Depending on its source, an illusion might be shared between multiple characters, having them perceive the exact same things.
However believable an illusion may be, it remains just that: an illusion. Therefore, many illusions are automatically disbelieved when interacted with. For example, if a character were to jump across an illusory chasm and fall in due to a failed Athletics check, the character will simply fall on the ground. Even if they lack any Discipline ranks, they do not have to roll to disbelieve the illusion after being confronted to its falseness.
Yet, many illusions require interactions to allow a check. If a character is searching a room which has a door enchanted to look like a wall, the character will be able to roll since they are looking for a way out of the room are likely going to examine said wall — merely standing in the room is not enough. In the case of implausible illusions, such as finding a giant ice cube in the middle of a hot desert, the character can roll a Discipline check to disbelieve even though they are not interacting with it.
Some interactions are automatic. For example, if the character finds themselves in a room that should be filled with a nauseous stench, the character are considered to be interacting with the illusion by breathing. In this case, the GM should allow a check.
In the end, it is up to the GM to decide whether a check is allowed or not and under which circumstances.
The difficulty is based on how real the illusion appears to be in this context. The more unlikely it is, the easier it is to see through it.
EXAMPLE OF A DISBELIEF CHECK
Nicoli is an imperial agent. A few days ago, he was investigating a suspected flesh sculptor who was seen in the area. Unfortunately, he was captured in the process and has been detained since then in a secret laboratory in an abandoned part of the city's sewers. The flesh sculptor has been testing many unidentified injections on him and Mark has been mostly sick since his abduction. At some point, he was able to escape his restraints while the scientist was gone. He picks up whatever useful items he can carry and heads out of the lab, finding himself in the sewers.
Ignoring his exact position, he starts to wander around hoping to find an exit. At some point, he hears a familiar voice. When he turns around, he sees his father. The man talks to Nicoli, telling him how he failed as an imperial agent, that he is not proud of him, and how there is no way he is going to get out of these sewers alive.
Nicoli's father is actually a hallucination caused by the drugs in his body. Nicoli's father actually left his family a years ago. He was never good to his son and was abusing him psychologically. The GM calls for an Average (dd) Perception check since the presence of his father is improbable at best. Yet, his impersonation is convincing enough. The GM still adds b to the pool since Mark knows his father has no reason to be there along with him.
The roll generates shh. Nicoli knows that his father is just a hallucination and is not intimidated by him nor does he feels restricted in his actions. Yet, the old man keeps talking to him and Nicoli finds himself arguing with him, and himself, just trying to shut him up. The GM spends a h to achieve this effect, which has Nicoli adding a b on all his checks while the drug's effects last. This is draining for Nicoli and thus the GM spends the other h to have him suffer a strain.