In qualitative research, the researcher is the research instrument. Its a live interaction. Researchers can unwittingly communicate their preferences, prejudices and biases. Fortunately there are ways of counteracting this and keeping the research valid. Part of the secret is to get into the right mindset before you start.
If you want to see what prejudices and biases you might have, try the Implicit Association Test. Its also an intriguing new research technique, used to assess nonconscious attitudes. Or use the process of Bracketing. Once you have become aware of your prejudices and biases you can then put them aside, in brackets (parentheses) while you interview. You can get them back later on!