Does anyone know when you write the affidavit letter at the USA embassy to be true and correct and get it stamped if you also have to show the document to the embassy that you are writing about example you state name wife name and date and marriage number, Do you also need to show them the marriage certificate? Thank you

I don't think they normally ask to see the marriage certificate when doing a affidavit stating it is true, correct and valid. Best thow to have it with you incase the officer wants to see it.

Can a lawyer go on your behalf to the embassy to get the affidavit done. I live pretty far from Bangkok or Chiang Mai. Thank you  

It can only be done in person. A consular officer office has to witness your signature and you have make a oath it is true and correct.

Edit: I merged your 2 topics. No need for 2 of them.

When I did that and any other documentation that was in Thai, I had to first go to one of the approved translation offices (STAR near the consulate in ChiangMai).  Then I had to take the documents to the consulate and they do a Notary Public document swearing to the contents of those documents.  That was many years ago so now I am not sure what their policy is.  Can ask on line or call them.

 

This is my recollection  of what I did 2 weeks ago to get an affidavit at the US Embassy in Bangkok. To answer your question is NO you don't need to show any documents. You can download the form from the US Embassy site and filled up yourself and get an appointment to go to the Embassy(good luck with that it took me 2.5 months to get my appointment because of the pandemic there given only 2 appointments per day).The affidavit is a sworn document you can prepare yourself with what is for and all your personal information and give to the officer at the window and you will have to take an oath in front of him that everything in that paper is correct an true and then he will sign and stamp. He may ask you what is that for to make sure match what you wrote in the paper. you have to pay $50. and you good to go. stay safe.... 

blank-affidavit-bangkok-january-2019.pdf