There's no such thing as a non-subjective (the word you're looking for here is "objective", I think) and fair interview. That's part of why it's so difficult. Everybody has a bad day--interviewers and interviewees. Some people can talk out a problem better than they can whiteboard, others the reverse. It is a wheelbarrow full of conflating factors and there's no magic bullet for it.

The tendency for engineering types to believe in "objective" processes for damned near anything that isn't measured with calipers is one of the bigger weaknesses in this field.