If it is a direct quote of someone, if someone is talking, or if a word is used ironically.
For example... If it were in a paper on Lord of the Flies, they might say...The personification of the island and the boys fear is shown when Piggy notes that, "And now that thing squats by the fire as though it didn't want us to be rescued-"... The paper there was directly quoting someone.
In a story if someone is talking.....The boys hurried through the inky darkness, fear pushing them faster. The boys paused for a breath, "I can't take this no more! We got to turn and face 'em!".....There, a character was speaking.
"Yeah, that "nun" you were talking with was wearing a bikini and had a pack of cigarettes in her boots!".....There, the nun, was put in quotations to indicate that the word isn't meant to be taken literally, eg. obviously the "nun" wasn't actually a nun.