Many illusion spells contain a statement about physical interaction with the illusion. From Minor Illusion:
... Physical interaction with the image reveals it to be an illusion, because things can pass through it.
If a creature uses its action to examine the sound or image, the creature can determine that it is an illusion with a successful Intelligence (Investigation) check against your spell save DC. If a creature discerns the illusion for what it is, the illusion becomes faint to the creature.
If one creature interacts physically with the illusion, does it become faint only to that creature, or to all who witness the interaction?
Please also address the case of an object not controlled by a creature doing the interaction.