I am planning to apply for a US visa soon to see one of IT conferences and from what I heard, in order to get the document I will need to meet one of US embassy representatives. Trying to imagine what such an interview looks like, I have to admit I completely don't know how am I supposed to behave then. For example, am I supposed to wear a suit, or are casual clothes okay? What are the topics I should not talk about?
It's called interview, as in asking questions rather than a meeting. Of course it might vary by the consulate where you're applying.
In my experience:
And that's it. Again, might depend on the consulate. In my case, not counting the wait time, I was at the counter maybe 5 minutes total, including 2 minutes of scanning all the fingers, which is tedious process.
I assume that you're applying in Warsaw, here is a forum post (in Polish) of someone who did it there.
Here's a complete list of instructions attached to the "appointment confirmation" document:
Judging by the document itself, you can easily tell that the security procedures are going to be annoying. Below I'll describe how the procedure looked like in my case - an interview in Warsaw. Please excuse the verbosity and feel invited to write a "TL;DR" section:
Since I didn't know anything what would happen if I happened to try to bring in one of the forbidden items, I decided to wear informal clothes and carry only absolutely necessary items before going through the security check.. When I reached the U.S. Consular Office, I could see about a dozen people waiting outside the office. I was 15 minutes earlier, but one woman from the crowd told me that she's just waiting to be let in because she had to turn off her mobile phone and that I'll probably be allowed earlier. While waiting, I could see a big sign next to the door. It said that it is strictly forbidden to bring any mobile phones, weapons and some other objects that I can't remember anymore. I felt worried, because the instructions said that I would be allowed to leave it at the security check. On the other hand, another sign on the door said that it's allowed to deposit turned off mobile phones.
A rather impolite man let in people one-by-one and when it was finally my turn, made sure that the phone is switched off by pressing a key, then took it and gave me a token. Then I went through the metal detector, the procedure reminded the one in the airports. A woman before me had some metal detected in her small bag and had it manually inspected, so if you plan to bring one, try to bring as few private things as possible. On the other hand, it was definitely bigger than an A4 sheet, so it looks like they aren't extremely strict when enforcing the instructions.
I was let in and after walking through a rather lengthy empty corridor (probably another security measure) and walking down the stairs, I entered the registration counter. My passport and some other document (either DS-160 confirmation page or appointment letter, can't remember exactly) were taken by a woman at the counter, which asked me for a photo because the one I uploaded while filling out the DS-160 document did not meet the standards - a bit of top of the head was not visible. I was then asked to leave, visit any local photograph and come back with a US visa photo. This made me realize that apparently they aren't that strict about the meeting times either.
One of the photographer stores was already closed, but the other one (still rather close to the U.S. Consular) was close as well and also offered instant photos. I had to pay 30 PLN for a one-off set of photos out of which one was then used at the registration counter, scanned and returned to me. Then, I was allowed to go through the next step of registration. At another counter, I had my fingerprints scanned (which, as vartec said, was indeed annoying because the scanner was really picky). After that, I was supposed to take a queue token and wait in the waiting room. It was rather comfortable, with access to a fountain sink (and of course a toilet nearby). Some videos about the US army were being played on both of the screens there. According to my token number, in about 15 minutes, 19 people were interviewed at one of the few interview counters there.
I think that my interview took much less than five minutes. I was asked about the information I entered in my DS-160 application - when do I expect to graduate, why do I plan to visit USA, do I have any documents about the conference I plan to attend (I didn't). Visa advisers warned me that I might be asked for additional documents since I was applying for a business+tourist visa, but actually, I didn't even need to show the invitation letter from my friend. There was also a question about whether I will be there alone and a question about my Krav Maga training that I mentioned in DS-160 (they wanted to know if I was being trained by an Israelite). The interview happens at the counter, in a standing position. The interviewer spoke with an American accent. After those questions, I was informed that I got accepted and I will be notified when the passport is ready for collection in the destination I chose during the online registration process. (here's another tip: check where that is! In my case, the location was very far away from the city centre and I would be better off if I didn't decide to save on the delivery)
A FINAL NOTE
This describes the process in Warsaw as it happened in my case, on May 6, 2014. It might look differently depending on how much time passed since me writing this answer, the particular U.S. Consular office and - of course - your case-specific issues. As dbkk said in his comment to vartec's answer, this might even differ A LOT.