One of the causes of the increased tension depicted in Act II, is the decision that Mr. Frank makes about answering phone calls. He makes that choice because he does not want anyone to find out that the building is occupied. If word came out, consequences would obviously be disastrous for the whole family, and their own survival would be in jeopardy.
The main way dramatic tension is built is the clues that Boo gives, making him interesting, but it can also be frightening. Also, the incidents that pop up in the story also builds up dramatic tension.
