What questions to expect in interview for US B2 visa?

First, some background. When you apply for a US visa, the officer is required to presume that you intend to become an immigrant, and it is up to you to convince him or her otherwise, that you have sufficient ties to your home country that you will return home as planned. We have a number of answers in our archives on how to show these ties, and I recommend you review them (for instance, this answer on bank statements, while talking about the UK, is quite helpful).

So the main issue your application runs into is that it is harder to show ties to your home country when you are asking to take an extended trip to the US. If you're able to spend so much time in a foreign country, you likely don't need to return to a job and don't have anyone at home relying on you. Therefore, it will be hard to convince the officer that you won't overstay your visa or work illegally in the United States. You should have a coherent explanation for your plans and the financial documentation to back them up.

Many websites give you an idea of what kinds of questions are typically asked at visa interviews. In your case, I would expect scrutiny as to why you want to spend so much time in the US, what you plan to do while you're there, how your personal circumstances give you so much free time, and your financial situation. Remember that US visa interviews only last a few minutes, so rambling or unclear answers will hurt your chances. Over-rehearsed answers may sound forced, and worse, false answers or documents could lead to much worse consequences, so always tell the truth.

Also note, as mts points out in the comments, that if you are denied the visa, you will be ineligible to use the VWP in the future and will need to apply again for a visa to travel to the US.

The main benefit a B-2 visa provides over the VWP is the longer duration of stay and the ability to apply to extend your stay or change your status while you're in the US. Many B-2 visas are issued as 10 year multiple entry visas, which give you greater flexibility, though admission to the US is always subject to the discretion of the officials at the border. There are also some legal differences in terms of your rights and status with the immigration authorities.

What are the benefits that B2 has over the waiver visa program besides the length of stay?

AIUI

1. The B2 allows "change of status", the VWP doesn't except in exceptional circumstances.
2. The VWP can only be used for land entries and entries by approved air and sea carriers. If you want to enter in a boat or aircraft that is not operated by an aproved carrier you will need a visa. If arriving by air/sea there is also a requirement for a return or onward ticket (though in practice fully-refundable tickets are probablly a more economical option for satisfying that than getting a visa).
3. Some nationals of VWP countries can't use the VWP. For example people who have overstayed in the past, people who have dual-nationality with certain middle eastern countries the US doesn't like and people who have recently travelled to certain middle eastern countries the US doesn't like. People with an arrest record.

The thing with 1 is that your visa should represent your intent when entering the US. Sometimes things do change but I would expect in most cases that if you say you intend to change status that things will not end well.

What kind of questions should I expect in the interview? And what would be a good answer for those questions?

I don't know for sure what they will ask but their overall intent will be to make a risk assesment as to whether you are a legitimate tourist who intends to return home at the end of their trip or someone pretending to be a tourist but whose real intention is to work illegally in the US and/or illegally immigrate.

In particular if your reason for getting a B2 visa is a tourist trip of over 3 months I would expect them to be interested in.

1. Have you planned out a workable trip.
2. Do you have enough money to complete the trip without working illegally or spending all your liife savings?
3. After your trip will you have a life to go back to?

Note that if you are caught lying to immigration or visa officials it is likely to very nasty consquences.