what shall i do during job interview?

First to your question, in ref. to monthly pay. Research the industry average and answer the question with "The industry average for a person in this position right out of grad school is xyz, but I'm sure your company has it's own pay scales which I will respect. You gave an answer but you didn't answer the question and you don't want to answer this type of question because if your number is to high you don't get the job if your number is to low you may get the job but the company would have paid you more. A big company has their set pay scales so if you are asked the question it's just a question to test you. When it comes to the interview itself remember you are selling yourself to the company and the company sells itself to you. I'm not sure about your country but it works like that here.