Many times when we are dealing with new people or who have not really had a talk we block. Even if you change a few words in the beginning, after a while appear silence, embarrassing silence.
What can we say to unfroze atmosphere?
First we should find a common ground.
For example: food, something about where we are, about the weather, something in politics, etc..
If you have some information about the person that talking, you could talk about topics familiar to him/her. If you do not know anything about the person in question, you have to put general questions, but questions that will not be answered consisting of only two or three words.
Very important is to show some interest on what the other person speaks. Even if you don't really care, pretend you're interested. If the person tells a joke - smile!, even an artificial one, even if you not understand it or not you liked.
If still nothing works - it means you are on different wavelengths. You have to admit this fact and give up before they turn everything into a culmination of awkward.