If your lucky to get a lunch interview there are some rules you should follow. First remember lunch interviews are more for casual situations. For example if your on your 2nd or final interview. Also lunch interviews are for people in executive positions.
The more relaxed atmosphere is a sign that your closer to the job. However you don't want to make a major mistake like poor lunch etiquette. They are watching you to see how you respond to a more relaxed environment so be prepared.
First of all order something on the menu thats simple. Something that lets you take convenient bites. Stick to water or a soft drink but never alcohol. Even if your interviewers order say a beer or wine I highly advise you to abstain.
Overall it is not a bad idea to stick to a similar dish or entree order of your interviewer. Of course wipe your mouth with your napkin and avoid anything that will make you pick your teeth at the interview.
Also be careful not to use your fingers. Though the company pays for the lunch be prepared to pay for both you and your interviewer just in case.
Now the interview itself involves you NOT talking with your mouth full. Also do not order anything with onions. Also only order desert if the interviewer does so.
This is one of the best types of interviews to have. The only thing is that you have to watch your etiquette and body language more than ever.