He will make something dangerously bad! As it will steal the character's mechanic and rewrite them as a dangerous mechanic. Natively, for example, Freddy's mechanic looks like this:

Freddy: He will go through the Right Hall (Floor 4). Watching him on the monitor will make him walk faster. Close the door on his face to save yourself from his attack.

Bonnie: Bonnie resides in Pirate Cove (CAM 05) with Foxy, Burnt Foxy and Lil' Devil Foxy. However, unlike Foxy, Bonnie will become even more agitated while being watched and he will jumpscare you if you look at him through the camera. Once Bonnie jumpscares, the camera will be disabled permanently. Checking the figure on your desk can help in knowing who is active on Pirate Cove.

Chica: Chica resides in the Kitchen (CAM 04) and shows her presence by knocking around pots and pans. However, if you hear nothing in the kitchen, then it means Chica has grown very bored of the music playing, and you can change the music. Once the music is changed, Chica will resume knocking around pots and pans to show that she's content again. She is enraged fast enough if you change the music too early. Once she is angered, she will leave the kitchen to jumpscare the player, by passing through the hallway and security door. Using the Global Music Box will soothe her.

Foxy: Foxy resides in Pirate Cove (CAM 05) alongside Bonnie, Burnt Foxy and Lil' Devil Foxy. Like in the original Five Nights at Freddy's, you can check him (but only if the figurine on the player's desk has changed to him) to slow down his attack a little, or else he'll escape and reach you. Once he's reached the office, he will disassemble himself faster, and his pieces will be slid through any open door or vent. Once all of his pieces are in the office, he will reassemble himself and jumpscare you.

You must reside inside the UFO to kill the pigs, or they will make all Characters go danger BRRRRRRRRRR.