When I used income I used to print online statements of my Thai bank account showing money in/out and online statements of my UK account showing my income going in/out..... don't think they were ever particularly interested. :)
I have a FCD with Bangkok Bank. The IO in Buriram made me take the book away and have it updated in Bangkok (Buriram branch did not have the facility to update the book) I had a standard letter from the branch and another letter from them which they said was "an update letter' because they not have a machine in the branch to update the FCD book'. No..... the tin soldier insisted on the book being updated and totally disregarded the 3 months of bank statements.