Direct discourse or dialogue should always be enclosed in quotation marks. In dialogue, a change in speaker is usually indicated by a new paragraph.
"And suppose his enterprise went wrong?" her husband suggested.
"It won't go wrong. Hasn't he made a success of his syndicate?"
"He says so - yes."
"Very well; then it stands to reason that he'll succeed in this too. He wouldn't undertake it if he didn't know it would succeed; he must have capital." (Howells, 18)