Documents required for B2 visitor visa for graduation day

Hi,

We are planning to get visitor’s visa for my parents and in-laws for the graduation day of my husband. Can you please let me know the documents that needs to be carried for the visa interview.

My father is retired government employee and my father-in-law is presently working as government teacher. Let me know what additional documents are necessary as proof of the employment.

Thanks

Santu.