I have a full grid, with all the across answers entered symmetrically. When the downs are entered, there are eight clashes, which are resolved in different ways. If I treat these cells as 'empty', then I have 14 empty cells.
I cannot understand the rubric about the three cards, but I suppose I could interpret this as saying that there are two of one shape and the third is the rest of the grid. However, the statement that there is something special about the last two letters in the first card eludes me; they seem to behave themselves just like everything else.
Now for the film. I love films and spend many days at the BFI, but I suspect that this is not a film I would particularly enjoy. However, the 14 letter fill is obvious, so it all boils down to 18 cells. Perhaps the eight discarded letters from the clashes are now used? But they are all over the place. So, as so often, it is a matter of guess what I am thinking.
I can, of course, see a thematic item, but you don't need an elaborate preamble to notice that. I will have to Google Xmas films, I suppose.