ok, thought so,
Try Iif(IsNull([tbl_lead_worker]![Role]) = True,Iif(IsNull([tbl_initial_worker]![Role]) = True," ",[tbl_initial_worker]![Role]),[tbl_lead_worker]![Role])
I hope the format is correct....haven't done this in a while, and it is off the top of my head....will check it out in a few...