I am not a lawyer, but...
Defamation requires 1. A defamatory statement 2. About a recognizable individual that is 3. published. Slander, the verbal equivalent of libel, also has requirement for actual damage to result, something you don't have to show for libel.
Starting at the beginning, what is a defamatory statement? This is basically anything that would tend tarnish the person's reputation and make it difficult for them to interact with others. Specifically, the sort of damages that the courts look for are the withdrawal or the withholding of opportunities the person would normally enjoy. Therefore, the slanderous statement is one that induces such withdrawal or withholding. It's no good to claim the statement was an opinion or even a quote from someone else, it's still defamatory for you to say.
Some statements, such as imputing criminal acts, infidelity, or loathsome disease to an individual are considered per se defamatory, that is defamatory without any other discussion. Other statements may only be considered defamatory given the circumstances of the statement. In your example, Snowden is accused of a crime, which is per se defamatory. Even assuming you're only using him as an example, I suspect the case you're talking about will still fall under the per se interpretation.
Which leads to the second element, who is a recognizable individual? This is the element TV always slips in the "Any resemblance between the events of this story and individuals alive or dead are entirely coincidental" line for. By explicitly stating the story is not about person X gives the author greater leeway when they later claim the story, well, is not about person X. This isn't a license to then actually make the story about person X, but it indicates a good faith effort to use the idea of the event and not the specific facts.
For example, it would be impossible to write about a black teen shot by a white neighborhood watchman at the moment without calling to mind the Zimmerman trial. But if you change key elements (make the teen younger or older, make the watchman female, set the events during a clear day instead of a rainy night, etc.), you break that automatic connection. It's still possible to imagine what might happen, just so long as you make it clear you aren't talking about what did happen. No amount of "This is fiction" labeling is going to let you say, "And then George Zimmerman did..."
The final element, publication, just means someone else heard (or read) your defamatory statement and understood it. There is debate in the courts as to whether that means they understood the statement or they understood the statement was defamatory. Either way, the whole purpose of a story or a novel is for other people to understand it, so it will likely be impossible to fight this element unless you write the story for yourself and then delete it without showing it to anyone.
If you are at all unsure about the law in your area, consult a lawyer before showing your writing to anyone.