I wish I could tell you exactly what is a “fair use" by another author of a copyright expression. I can’t and neither can anyone else. It is a vague defense to infringement allowed by 19th century judges, and eventually put into concrete words by Congress. The words are so concise they are not so precise.
But I am tough so I will give it a try. As you read this please keep in mind that the judges always—when fair use is claimed—try to compare two stakes and their importance. One stake is of the original author, who is entitled to control copying and distribution of copies of his or her work. (That could be text, music, visual art, animation, live action video or any other medium.) That stake includes the right to expand the original and make “derivative works," such as a movie sequel. The second stake is the second author, who has a stake in making some use of the original, because use is how expression develops. The law accepts that development.
When it codified the fair use defense in 1976 Congress said:
“In determining whether the use made of a work in any particular case is a fair use the factors to be considered shall include—
(1) the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;
(2) the nature of the copyrighted work;
(3) the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
(4) the effect of the use upon the potential market for or value of the copyrighted work." [US Code, title 17, section 107.]
Look at the first factor, the character of the use.
Now the second factor, the nature of the original.
The third factor focuses on what portion of the original is used.
The final factor is economic, effect on the market for the original.
There are two more important points not in what Congress wrote. First, a judge and jury are likely to consider, perhaps unconsciously, whether you are a good guy or a bad guy. Second, giving credit to the original author buys you nothing.
The short of all this, sadly for creative people, is that fair use is messy business. Even lawyers and judges disagree about exactly what it means. If you have any doubt at all, you had better—before you publish—consult with a copyright lawyer, who is familiar with all the cases decided both before and after Congress spoke. He or she may well need to look for cases close to the one you present and how they came out, and lawyers learn in law school how to do this effectively.
© 2012 Philip L. Marcus.