Camera, tablet, cellphone, or portable scanner to digitize documents and photographs (obtain permission first, and get a release form signed if you plan to use any of it for publication)
Recording device such as voice recorder/camcorder/cellphone. (Ask permission before using.)
After the interview process
What's next?
Make sure to organize all the information right away.
Save all your research in several places--make sure to digitize it as well.
Keep a record of who and where you've search for information.