The stronger an Illusionist, the more realistic their creations will be, and the more the people seeing them will believe in them.
Often, an Illusionist specializes in creating hallucinations of a few specific senses, instead of all of them. Many choose to specialize in sight or sound, as those senses also allow them to trick humans, but those with the power and the focus generally remember that werewolves rely on their noses as well.
The greater the size of an illusion, the more power and focus it takes to maintain. The same goes with time. The longer it remains, the harder it is to keep it strong and believable.
Gifted and trained Illusionists can create illusions so real that a person might believe a dead loved one has returned to them.