Take a moment to read this tutorial on good interviewing practices from the Journalist's Resource (at Harvard's Shorenstein Center on Media, Politics and Public Policy).
David Poulson at the Knight Center for Environmental Journalism has a good list of tips for interviewing.
PBS News Hour's Margaret Warner says she starts preparing for an interview by checking secondary sources like the BBC, the Christian Science Monitor and The New York Times. But that's just the beginning: Then she checks primary sources, like the Supreme Court briefs for a legal case, or interviews with experts if she's heading to another country to report (from the Journalist's Resource).
Do you need to conduct interviews for an investigation? Here are some good tips for interviews that often take more thought and preparation (from Exposé: America's Investigative Reports, produced by the Center for Inestigative Reporting and Thirteen/WNET New York).
Children are vulnerable interview subjects, and that means journalists need to take pains to be fair when interviewing them. Here are some helpful tips and guidelines from the Dart Center for Journalism and Trauma.
Watch this video about how to video a formal interview (from the International Journalist's Network).
INVASION OF PRIVACY
The Reporters Committee for Freedom of the Press provides a good overview of invasion of privacy, including intrusion and private facts.
Here's a good discussion about publication of private facts from the Digital Media Law Project. (DMLP is hosted by Harvard University's Berkman Center for Internet & Society).
States differ about sources' rights to know if you're recording them, but here's an overview about the laws governing recording interviews (from the Journalist's Resource).
ELECTRONIC COMMUNICATIONS PRIVACY ACT
This law, passed in 1986, mainly deals with communications over the telephone or Internet. So, for instance, this is the law that says it's illegal to hack someone's cell phone.
For some basics, read the information at this site from the U.S. Department of Justice.