How much detail to enter for the original tax return prepared by someone else when preparing an amended 1040?

If you are going to e-file the amended return, you need to enter pretty much all of the detail so it walks and talks like the original return.  If you paper file the amended return, as long as you have correct totals on the various lines of the 1040 you can shortcut the system to get the return done.

I "think" if you prepare the correct return first, THEN activate the 1040X,  you can then just go to the 1040X and put the figures from the original return directly into the Original Column of the 1040X and it will Efile....you dont have to prepare the return exactly as the original was....that being said, in order to prepare the amended (correct) you will need the all the details for the correct return, you cant just fudge them in anywhere.

I tried this on a dummy return and it didnt give me any errors for Efiling.

If I didn't prepare the original return, I usually tell potential/new client the fee is my regular base fee not my lower amended fee as I pretty much do the return wrong first to enter as much as possible. Plugging and overriding if I don't have the actual original paperwork. Once I know I tie out / match the original return I save that file called original (or input to amend). Then do save as and do the amended return checking off the box original return was accepted. That always works for e filing for me.