Interviews and observations are field study methods like contextual inquiries.  In both cases, you should visit the participant "in the wild" to better understand how they get their work done in the context of their work.  

Field Study Tips

These tips are relevant for any type of user sessions you run but are particularly relevant for interviews and observations where you typically meet with individuals.

From Jakob Nielsen, Field Studies Done Right: Fast & Observational




For more on interview technique see:

Nondirected Interviews: How to Get More Out of Your Research Questions

Face to Face With Your Users: Running a Nondirected Interview


"Users are perfectly capable of expressing their latent needs.  They just can't do it verbally.  That's why we do ethnography and empathic research" - Rich Sheridan, Menlo Innovations



For more on observation technique see:

Usability Net's definition

UIE Seminar: Field Studies: The Ultimate Tool in Your Usability Toolbox - purchase for $129