In the AfterUpdate event of that control, you can use the DLookup to locate the supervisor's name and place it in the locked control
It should look something similar to the following (after correct substitution) ...
Private Sub txtControlName_AfterUpdate()
Me.txtLockedControlName = Nz(DLookup("[SupervisorFieldName]", _
"tblTableNametoFindSupervorNameIn", _
"[ForeignKeyinCurrentTable] = " & Me.txtControlNamewithKey), _
"No Supervisor"
End Sub
Again, this is air code, but should get you going (check the Help for DLookup, too). I added the Nz function because Access will wig out if DLookup returns a null value, so this should handle it for you.
HTH,
-dK