Just because you are supposed to be tactful, it doesn't mean you should be afraid to express differences of opinion or even profound disagreements with people - and that includes interviewers!
Tact is the ability to say or do things in such a way that you do not leave people feeling unhappy or angry: it is the skill of dealing with others sensitively when difficult or controversial issues are being discussed or need to be raised.
To help you to be tactful even when you are in disagreement you need to read the other person's personality and treat them accordingly.
Here are four useful tips.
1. Use the Platinum Rule - treat other people the way they want to be treated.
2. Take the time to respond to other people rather than instantly reacting.
3. Don't be afraid to offer a critique but stay away from criticism.
4. Try to be empathetic rather being overly sympathetic.