I play a character who suffers from both drug-related and PTSD hallucinations, and I start off by writing how he's reacting to the hallucination but not what he's seeing. If this comes off as jarring to the other player, well... good. It probably would be rather unsettling to see someone recoil from something for no apparent reason, and it gives the thread some intrigue. Depending on just how much I want to reveal to the "audience", I may never detail exactly what he's seeing. The only thing the other player actually has to know is what my character's doing, so that they can react to that. Sometimes giving more than that is giving too much, especially since you can get several threads out of other people asking what your character saw, etc., and to be repetitive would be boring. If you do feel a great need to go into detail about these visions, modifiers like "in his head" or "he thinks he sees" or whatever will usually be enough of a hint to the other player at the start so that you don't need to mention them every subsequent tag. If you're the type that feels the need to go on for paragraph upon paragraph describing visions, italics or blockquotes would be a further indication that this is a story within a story.