The mood of a piece can change based on the situation, character development or their understanding of their circumstances. It is the feeling the reader gets from the writing.
Mood is created by vocabulary selection and also by the length/structure of individual sentences.
Whether you are trying to portray the mood as:
creating the proper mood will draw the reader in but it may take a few additional twists to keep them there.
When I am hunting for the right word, or inspiration of any sort, I head over to Becca and Angela's blog - The Bookshelf Muse. They have collated the widest collection of words to empower your writing.
Happy Writing... or intense or frightening or tragic or magnificent! ;)