Several times now we have brought up the issue of Morgoth's lust when it comes to the dancing Lúthien. This is primarily based on the description of Morgoth in the text of the published Silmarillion during that scene. Naturally, this means nothing good, but is not overly specific.
”Then Morgoth looking upon her beauty conceived in his thought an evil lust, and a design more dark than any that had yet come into his heart since he fled from Valinor.”
His words to her in the Lay (1930) make it very clear what he intends to do with her...and that's not very ambiguous, though still more circumspect than you might expect such a clear threat to come across.
"Yet I will give a respite brief, a while to live,
a little while, though purchased dear,
to Lúthien the fair and clear,
a pretty toy for idle hour.
In slothful gardens many a flower
like thee the amorous gods are used
honey-sweet to kiss, and cast then bruised,
their fragrance loosing, under feet.
But here we seldom find such sweet
among our labors long and hard,
from godlike idleness debarred.
And who would not taste the honey-sweet
lying to lips, or crush with feet,
the soft cool tissue of pale flowers,
easing like gods the dragging hours?
A! Curse the gods! O hunger dire,
O blinding thirst's unending fire!
One moment shall ye cease, and slake
your sting with morsel I here take!"
In his eyes the fire to flame was fanned,
and forth he stretched his brazen hand.
It is worth pointing out that lust is being used in this story as the opposite to love, and so the focus and emphasis is on using someone as an object for one's own purposes and desires, with little concern for their own desires, their good, or even acknowledging them as a person. Lúthien is being compared to flower petals, an object for use, and one that can be taken against her own will.
Rape involves sex, but it is about power, about controlling someone else. And, while it is clear that whatever Morgoth intends to do to Lúthien will have a sexual component to it, what he is actually going to do is completely wreck and destroy her. She's not going to survive whatever he's going to do to her. Not sure why I feel the need to point out that the threat of murder is less veiled than the threat of rape, but whatever, they are both very clearly intended. First torture, then death.
So, how do we prepare the audience for this scene? Morgoth isn't exactly sitting around Angband watching snuff porn, which...doesn't exist in Middle-earth, not even in Angband. Tolkien focuses Morgoth's attention on what the Valar have in Valinor that is denied to Morgoth at this time - they have an easy life, full of sweetness, whereas his is dominated by work and drudgery. Life in Angband is ugly compared to life in Valinor, and the repentant Melkor lived in Valinor for years. We can have him express some bitterness for what he has lost, some jealousy of his brother Manwë.
Silm Film did include the love triangle between Melkor, Varda, and Manwë. So we can make use of that, reminding the audience that Morgoth had wanted to possess the light of the stars for himself.
Silm Film did not include the story where Morgoth has the history with Arien detailed in the 'Myths Transformed' section of
History of Middle-earth. So, in Silm Film, Arien is a proto-balrog, an angelic follower of Melkor in the beginning, who defects to the other side when the balrogs decide to tear down the Lamps. The destruction of the Lamps is the action that locks the balrogs into their monstrous forms, and so Arien escapes that and chooses to join Varda instead.
I am not sure what details we want to include to point at the direction of Morgoth's thoughts, but I agree that it should not be a sudden realization that, hey, this elf girl is really hot, I wanna watch her dance a bit....