You know one or the entire panel of interviewers

Ignore this fact, do not start calling people by their Christian names. Remain professional and answer their questions as if they had never met you before.

Badly prepared interviewers

Be patient and not critical of their incompetence.

Questions you don't know the answer to

It is better to say, "I'm sorry I don't know the answer to that question" rather than to 'Um and Ah' a lot and say a lot of rubbish.

Questions where you are asked to give an opinion

Give a balanced view, offering both sides of the argument. If you know what the company policy on the matter is, perhaps go with that, or if you can explain articulately why that is the wrong policy to adopt.

The feeling the interview outcome is already known

Do not let this effect your performance, you never know for sure what their decision will be.

Deciding during the course of the interview you are no longer interested in the job

Do not get up and leave, continue with the interview as you would if you wanted the job. If there is a specific problem you have that has changed your mind discuss it, perhaps there is a way around it?

Seeing another interviewee who you know

If you meet a friend in reception, exchange the time of day but do not be too chatty, in this environment they are the competition. Remain professional and wish them good luck with their interview.

Interviewers are not ogres, most of the time they are not trying to catch you out they simply want to get the best personnel for their company. If you stay cool and calm and follow these guidelines you won't go far wrong.