An interviewer's attention level can be nearly impossible to detect, because most people are skilled at nodding their heads and saying "hmmm" while looking at you, all in an effort to disguise their wandering minds. The longer you speak without interruption, the less attention the listener is giving you. Hence, when you provide a long interview answer that builds to an important conclusion, often the interviewer is no longer listening. This is particularly important when you respond to an interviewer's request to tell him about yourself, because there is just so much you can say on the subject, and you can't be sure what part of your background the interviewer is most interested in learning about.