Personally, I like to think of it like I'm the one interviewing the company. I always have questions ready to ask the interviewer to learn more about the position and the business. I like to have fun with it too. I always smile and throw an occasional joke. If they laugh at your jokes, you got the job :P
That is true, I am quite the subtle joker in interviews myself. I think the key is as you say, be relaxed and engage with them