In this article, we took a general look at fair use, the 4-factor "fair use" balancing test, and an example dealing with fair use. As you can see, fair use can be somewhat of a grey area in the law, which changes based upon the specific circumstances. So, keep in mind that if you plan to use the copyrighted work of another, it pays to see if it clearly falls within the fair use doctrine prior to using the work.
Also, remember that fair use is a defense to copyright infringement. As long as the copyright owner does not take any action against you, nothing will likely happen. However, if a copyright owner finds out that you’re using his or her work without his or her permission, the copyright owner could threaten to or sue you for copyright infringement.
In court, the copyright owner would have to show prima facie evidence (i.e. a basic level of evidence) that you committed copyright infringement. If the copyright owner can show such prima facie evidence, then you could attempt to use the "fair use" doctrine as a defense to copyright infringement.
Finally, as previously stated, it’s generally best to obtain written permission from the copyright owner prior to using his or her copyrighted work.