This is a tough one, because if you go to any job hunting advice site, they'll have a dozen different rehearsed ways to "be natural."
When I was a recruiter, I had a candidate who read that he should make sure to smile, so at the end of each question, there would be a pause, and then a huge, forced, fake smile that really creeped-out the interviewer.
Probably, if you make a point to try and briefly make eye contact with the different people (briefly, for each individual, changing the target of your focus as you talk so you're not staring down just one). Look at the people you are talking to, but don't stare, in other words. Then you are talking to them, not talking down at the table or avoiding them.
It's a mild form of interaction that wouldn't necessarily force you to run contrary to your normal personality, but would engage you directly with the interviewer(s). This will also force you to observe their reactions, which can be helpful.