I think the best approach should be for Drill-Down report is one liner expression as shown below.
=DrillThroughProvider.DrillInvoice(Parameters!AX_ReportContext.Value, First(Fields!InvoiceId.Value, "SalesInvoiceDS"))
Where in "DrillInvoice", "Invoice" is created "FieldGroup" in respected table "CustInvoiceJour" which contains "InvoiceId" along with other fields. Other fields doesn't matter, only "InvoiceId" should be there.
What you need to, do everything as it is to fetch information related to particular field. In properties "Action" tab write the above expression according to your field. This is nothing but expression to fetch the information from DS.
First(Fields!InvoiceId.Value, "SalesInvoiceDS" (Header)
or
Fields!Name.Value (Line or table)
