Hmm It has been a long time...
since my first H1 but be sure to carry these docs:
1) I-797 approval notice, I-129 and LCA ceritified copies ( these must be sent to you from the US company ).
2) Your degree certs/transcripts/Degree equivalent to US degree certificate.
3) Previous letters of experiance. Originals.
4) Previous appintment letters. Origianls.
5) Resume, same as that was used to apply for the H1.
6) More infornation about the US company, get product brochures, payroll information, income tax records..if you dont have these ask the company to send it to you.
7) Mumbai is a better place to get a visa than chennai...if you are applying for a software position they sometimes ask you about your company...type of work you will do over there..I have heard rumors of people getting asked technical Qs ( but that is only for the shady positions that pay $36K per year with skills in database and cobol ) and I seriously doubt if any visa officer has any idea about what engineers do...my case for hardware digital design ..no questions asked at all.
Speak clearly, slowly, if you dont understand the accent please ask them to repeat the question. relax...you will get it.