Make them wonder what you're thinking or feeling, but never actually tell them. This will keep them on their toes and make them question their own sanity.
Example: If they ask you to do something, respond with a 5-minute pause before answering, making them think you're considering it, but really you're just deciding whether to ignore them or not.
Example: If they ask you to talk about something, suddenly change the subject, or better yet, pretend you're interested in something else entirely, like your favorite video game or a new recipe you found on the internet.
This will make them feel like they're in a relationship with a human version of a coin flip.