Measure for Measure reaches its height of tension early, with the encounter between Isabella and Angelo and the issues that their meeting raises. Angelo find himself suddenly vulnerable to the same sinful desires for which he is having Claudio put to death. This changes his position completely; no longer on a moral pedestal, he must instead spend his time avoiding culpability rather than carrying out the law.
Lucio seems to comprehend Angelo's vulnerability from the start, encouraging Isabella to touch him and be less cold. Lucio is encouraging Isabella to exploit her femininity to convince Angelo. In a way, he is even encouraging her to offer herself as his sexual object in order to save her brother's life. Lucio may well know that Angelo would respond by propositioning her, and he may expect her to accept, just as her brother will when she explains the dilemma to him. Only Isabella understands fornication to be a deadly sin, which is why the thought is so repulsive to her.
The Duke enjoys his newfound power to absolve sinners as a friar. He shows natural sympathy towards Juliet, and it is clear that he would be more merciful in Angelo's place, but that he is not against Angelo's actions. Already we see the Duke's desire to operate power from the inside, investigating the various characters in his disguise and determining from the evidence they provide what the best course of action will be. The Duke is the only character who appears in almost every location in the play; his hand is active everywhere, and he is pulling most of the strings.