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

Interviews

Strengths

Weaknesses

For more on interview techniques see:

Observation

"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

Strengths

Weaknesses

For more on observation techniques see: