You should only spark up a conversation after someone smailes and acknowledges you. Do not interrupt two people in order to discuss something unimportant such as the weather. If someone is reading a book or writing a letter at the bus stop it is not appropriate to initiate a conversation either. Another good time to make small talk is during a break in a meeting or presentation when there is nothing important going on. Finally, it is important to recognize the cue when the other person wants the conversation to stop.