The nature of the copyrighted work.
Fair use is more likely to apply if the original is a factual work.
The amount and substantiality of the portion to be used in related to the copyrighted work as a whole. The larger the amount, and proportion, of original expression you want to use, the less likely it is covered by fair use. The effect of the use on the potential market for, or value of, the copyrighted work. Educational fair use says you may copy an excerpt one time, as long as you make no more copies than there are students in the class. The quality of subsequent copies, furthermore, does not degrade as it does with older media.