An example sentence with the word "demeanor" is, "John operated with a professional demeanor." This word is a synonym for manner, conduct or behavior, so it works as a substitute for those words in a sentence.
Another example is, "I don't like your demeanor in the classroom." A teacher may speak this sentence to a student to let the student know that his manner or conduct must change to fit the classroom standards. If a person was upset, someone observing that person might say, "Jim tried to hide his feelings, but his demeanor and facial expressions clearly showed that he was upset."