Smirnov did not expect Popova to be the kind of woman who would defend her dead husband's honor by challenging him to a duel. 

Smirnov is totally undermined, he falls for Mrs. Popova, and ends up in her debt.  Smirnov is a hypocrite, he claims to be a brave man, yet when Mrs. Popova challenges him to a duel, he is afraid.  She, on the other hand, is brave and unflinching, she wants to get rid of him, so she is ready to shoot him. 

"POPOVA. Are you afraid? Yes? Ah! No, sir, you don't get out of it! You come with me! I shan't have any peace until I've made a hole in your forehead . . . that forehead which I hate so much! Are you afraid?"

"SMIRNOV. Yes, I am afraid."

"POPOVA. You lie! Why won't you fight?"

"SMIRNOV. Because . . . because you . . . because I like you."

"POPOVA. [Laughs] He likes me! He dares to say that he likes me! [Points to the door] That's the way." (Chekov)