I submitted I-797 original and letter of employement from my employer . I didn;t take any document from the client.
I took copies of all the documents provided by employer which they had submitted while petitioning for my H1B. This my employer provided when  I asked them for document as I am going for stamping.
LCA
I-129 and all other which they had used while filing.

in addition to that I took tax return/pay stub/bank statement showing activity for last 6 month.

In the interview he asked

Where do you work?
Client location
What do you do?
Described what I do
How many employee your company has?
Replied how many they had. This is provided on I-129
I mentioned what I did and also that what we do for client company.
What is your title?
How many hours?
40 hours and extra hours to meet deadlines. If you would say less than 40 hours this could raise flag
Who do you report?
You should say that you report to your manager in your Employer ( Consultant) . That is how our team is structuted. Don;t mention your client when you are asked who you report to.
How much you making?
I told whatever I make,
Were your previous employement contract based?
No, I had worked previous as employee for two other companies.
Last he saw my previous student VISA and asked so u studied here.
I replied back saying I went there but I got transferred to other university and completed my study there.