If you are interviewing for a job and the interviewer has set his interview in a local restaurant during the meal. You need to dress accordingly and try to reach the place early and find a right spot and relax before the interview.
The interviewer can suggest a business meal in the second interview with them, as they will want to know how you behave in a restaurant and your social skills outside of the business environment.
They will observe your every move and you know that you are under strict vigilance. When you order, food or drink be polite to the waiters or any other person taking the order, as this will show your people skills and leadership skills. You will be put to test and the interviewer can judge how well you will behave with the subordinates in the office.
If you know you cannot carry alcohol avoid it and try to stick to the water. However, if the interviewer is ordering an alcohol then may be you will also want to do that.
Try not smoking and especially if the interviewer is not a smoker avoiding it completely.
Your order should be plain food and nothing messy like spaghetti and never speak with your mouth full. Also avoid anything with the bones if you are ordering non-vegetarian ask for boneless. Avoid expensive items on the menu. Avoid sending back the food you have ordered,
If the bill is kept towards, you do no take it. The interviewer had invited you for the meal hence it is his duty to pay. You can just thank him/her for the meal.