I can never figure out why people add questions in topics that have nothing to do with the question. :/
Anyhow, to eavesdrop, wait for the guard to get into a conversation with someone (or if you have an other "party member," which I think you do at that point, you can "cheat" and have them start up a conversation with the guard.) Then click on him and "eavesdrop." --- Please insert witty quote here.