One constant theme is that the Defense Against the Dark Arts teachers are all short-timers, if not downright villians. Dumbledore could make Hogwarts safer if he just discontinued the class altogether. There must be a reason he wants to provide a place for n'er do wells.

In any event, I didn't trust Lupin on the train because I didn't believe he was really sleeping. Then when the dementors came around, I thought it strange that they didn't go for Lupin. Surely an adult would have more unhappy memories than a child. When Harry ate the chocolate and didn't turn into a blue toad or anything, I figured maybe Lupin was okay.

That lasted until Lupin took Harry's map away from him. Then I wondered again. At the end, when he was leaving Hogwarts, I was sorry to see him go.