Life span differences

Rob Conley's bit about elves in his recent examples of cultural motivations is interesting:

Then there are Elves. Immortal they provide a living memory back into the depths of time for the cultures they come into contact with. It as in the 21st century we have people who knew and spoke with Julius Caesar, Charlemagne, and other historical figures of the past.

This would be an issue for any two races of significantly differing lifespans. How they each deal with the issue would indeed tell much about their cultures. An elven culture might, for example, have some sort of "prime directive" to prevent them from derailing the cultures of shorter-lived races. Imagine a group of humans who, in order to extract valuable information, hold captive a memeber of a long-lived race—perhaps for generations.