Many Indian restaurants are not very formal. Yes even those considered best (Indian restaurant are usually very similar). The etiquette for Indian (or any ethnic South Asian) restaurants might be little different from other restaurants. Below I have provided some basic rules.
1. No Beef or Pork ; Many Indians are either Hindus or Muslims. To Hindus cows are sacred, so no beef and to muslims pigs are disgusting animals to consume, so no pork. There are a few restaurants in India which are pure vegetarian (They dont even serve eggs).
2. Eating with hands ; Many types of foods especially the indian breads are consumed by hands. It is ok to consume food with hands provided your hands are clean and the food is not messy.
3. Eating from others' plates ; Indians have this concept of people not encouraging the sharing of plates or cutleries coming in contact with their food. You cannot consume food from other plates beacuse of the saliva or the indirect contact of your mouth into someones food.
4. Wining and Dining ; Indians do not have wine and dine culture and so even the gevernment restricts eateries serving liquor without permission. Be aware not all restaurants serve liquor