Plagiarism comes up a lot when I'm in school and writing papers. It got me to thinking.
See, I'm making an animated film and I'm getting one of my friends to record a guitar part for one of the scenes. The guitar part is based off this:
at about 2:46 this really haunting guitar riff starts up and it lasts only a few seconds. When I first heard this I knew it would be perfect for the scene if it was elongated and reworked... and I got with my friend and told him that.
So, my question is... am I plagiarizing? do I have to credit Skip James? or is this an "original score" since well it only uses the idea of that riff and doesn't copy it exactly.
Elongated and reworked. Just make it your own. Change a few notes, syncopate it a little more and don't worry about it. You probably have some ideas of your own anyway..put them in there and go for it.
Although any blues scholar would probably be quick to point out that plagiarism has never been much of a sticking point in the history of the music, it sounds like you've been trained in the Western academic paradigm of intellectual property. Fair 'nuff.
In the academic sense of plagiarism, I find this is a useful way of deciding whether or not it is appropriate to cite an idea:
Plagiarism is, obviously, when you deliberately take someone else's work and present it as your own. Here it's common decency to cite the original craftsman.
But it's perfectly acceptable to borrow someone's idea and build on it until the idea is as much yours as theirs. In this instance you're not really stealing, but it's courteous, as well as helpful, to use citations to inform the reader of which ideas are yours and which belong to someone else.
Of course, as Led Zeppelin among others have inadvertently shown, the culture of citation is not as rigorous in blues music as it is in academia (Willie Dixon certainly would have had something to say on this topic). At any rate you're not going to insert a caption saying James, S. 1936. "Devil Got My Woman" when the riff starts up. Needless to say, you'd look a fool.
Perhaps you can acknowledge James in naming the song (much more common practice in music), such as calling it "Devil Got Skip's Woman" -- or something along those lines. If you are drawing on his riff, that would be honest and respectful without being at all inappropriate or awkward.