What character traits make Fortunato such an easy prey for Montresor?
Edgar Allan Poe's short story "The Cask of Amontillado" is a tale of murder and revenge. The story depicts the revenge of Montresor against Fortunato for "the thousand injuries" he committed against him.
Montresor chooses to enact his revenge against Fortunato during Carnival. Carnival is a time, typically in February, in which people are able to celebrate excessively prior to lent. Many times, people go to extremes in partaking of alcohol and food. People also dress up and wear masks. They are able to be "something" or "someone" else during this period of celebration without worry of criticism.
Overall, Fortunato possesses two "traits" which make him an easy prey: being naive and being drunk. While both are not character traits, unless you consider Fortunato to be an alcoholic, each allow Montresor to lure Fortunato to his death.
Fortunato is drunk, given it is Carnival. He has a love of great wine- which is how one could justify naming him as an alcoholic. Since he is drunk, and a lover of great wines, Montresor is able to insure that Fortunato will follow him into the catacombs.
Second, Fortunato is naive. If he did enact a "thousand injuries" against Montresor, he should be more weary about Montresor. Instead, he trusts Montresor completely.