Casanova, take 15ac, for example. The central cell has two letters, and the diagonal line separating them must be drawn from the NE corner of the cell to the SW; one letter goes in the NW triangle, one in the SE triangle, then 8dn will read correctly.
On the other hand, in the last cell of 19a the diagonal line runs from NW to SE for 12dn to read correctly. The last letter of 19a precedes the penultimate letter in the word entered for 12dn.