For example, if you enter or leave a shop you should say Dobrý den/Dobrý večer (Hello/Good Evening) or Na schledanou (Goodbye) respectively. Obviously in larger shops such as hypermarkets you don´t need to greet people as you enter, but you should certainly acknowledge the person at the counter. And if you´re sharing a table with strangers in a restaurant or pub it´s common courtesy to greet the people sitting there. The same goes for lifts, and also compartments in trains. And in some ordinary trains, which usually don´t have compartments, you´ll notice people saying goodbye to each other if they´ve had some form of contact, whether it be a brief conversation or if one person has helped another in some way, e.g. with a suitcase.