what kind of questions do they ask in the fiance visa interview?

Answer 1

It can be a wide range. They might just ask to see your tax return and a few "chit-chat" questions then stamp up your passport. They might enquire into your relationship a bit. The officer will need to be assured that you do, indeed have a genuine relationship. They may well separate you, ask the same questions and compare answers. They might ask what you see when you walk in your front door. Or what you had for dinner last night. Maybe what was the last gift you bought each other.

The questions are designed to identify those people who are not in a genuine relationship. If you are, you have nothing at all to worry about.

Answer 2

Some general questions they may possibly ask are:

How long do you know your fiance?
How did you meet?
Have you ever been to the US?
When is the last time you have seen your fiance?
When will you leave for the US?
When did you meet in person?
When did your fiance propose?
Why was your fiance's first marriage not working out?
Does your fiance have any kids? How many, how old, who do they live with?
When did your fiance divorce?
What is your fiance's birthday?
Does your fiance have any brothers and sisters?
Where do your fiance's parents live?
What does your fiance do for a living?
Where does your fiance live?
Where was your fiance born?
What do you love about you fiance?
When are you going to leave?
When are you going to have the wedding?
Have you ever applied for a traveling visa?
Where did your fiance visit when he was here?

They also sometimes request to see pictures of you together, letters/cards/e-mails sent to each other, phone bills showing you talked to each other over the phone, etc.