Once you have been invited for an interview at meal time, there are certain things expected from you. Besides table manners you will have to conduct yourself in a proper manner because you can rest assured your boss will be watching.
When it comes to ordering, let the boss order first. And order something simple and cheap for your self. For god sake do not order a meal like a lobster where you will have to use your hands to eat. A salad and/or soup are the easiest to eat and least sloppy. When it comes to drinks, just stick to water or simple beverages like tea or coffee. Do not order alcohol or wine.
When it comes to your food, just simply eat it and be quiet. Do not complain about the freshness or style of presentation. Remember this is where your boss eats and he/she may not be pleased. When asked questions, finish chewing first and do not speak with your mouth full. The interview is not to check your appetite but your behavior. Do not slurp your soup and never rest your arms on the table. When finished wipe your mouth hands and place your napkin back on the table.
There is no real magic about eating or the interview. If you use common sense and have decent table manners, the rest is easy