Lighting: The scene sets the characters shrouded in darkness, yet they're well lit. This high contrast between the almost glowing figures of Spike and Buffy is established through high key lighting, where the windows on the sides of the church serve as pseudo-diegetic sources for light. No matter where Buffy and Spike stand throughout the scene, they seem to fall into the light. Save one exception: when Spike willfully crawls into the shadows to confess his darker passions, of death and sex ("I dreamed of killing you, I think they were dreams"/"Thinking of you and spilling useless buckets of salt"). When Spike finally confesses outright that he has a soul, upon saying the word "spark" after hinting at how he's like Angel, he finally walks back into the light. The scene juxtaposes light and dark at its most extreme, the contrast between the demonic and the soul.
Color: Consider the lack of warm color in the scene, as it's primarily blue-toned, save for the few candles at the walls firing orange. This cool color palette complements the high contrast between the glowing white figures of Buffy and Spike (their blond heads, their pale skin) and the darkness that surrounds them. The spark within Spike is meant to "burn" yet the colors in this scene are lacking fire ("Ain't we in a sodding engine?); this color palette will be dramatically contrasted in Chosen when Spike finally feels his soul as he lights up to close the Hellmouth, his final scene with Buffy awash in warm warmth (bright white light creating warm oranges and reds).
Costume: Buffy's clothes fit this blue color design. Perhaps more interestingly, Spike begins wearing this garish blue shirt which he sheds once he's in the church. Upon doing so, his costume fits the dichotomy of light and dark. Black jeans beneath (baser passions), his chest baring glowing pale skin (where the heart resides, the soul). He's as dramatically light and dark in body as he is in spirit, as the scene itself is lit. There's an irony here in how Spike's bleached hair, the bad boy persona, is this pure white color on top of his head -- within this color dynamic of light as good and dark as bad, the artificial color of Spike's hair seems almost like an indictment of his attempts to be bad. Even when he's bad, he's still coloring himself in good.
Sound: So much of the sound in this scene is simply the characters talking. There's the ominous music playing softly underneath it all, the string instruments humming deeply. But it's the voices that really decorate the scene and I'm not sure how, but somehow the actors sound as if they're whispering even as their voices remain clearly audible. And what's more, there's an echoing quality to their speech, a resonance that further builds the sense of emptiness surrounding them. They're surrounded by darkness and their voices echo. This effect makes the distance between Buffy and Spike seem even greater; they're alone even as they're standing together in the same room, even as they're speaking directly to each other.
When Spike explains why he got his soul, "To be a kind of man," the music is briefly hopeful, the strings light and higher. Then the music drops back to the low ominous sounds as he says, "And she shall look on him with forgiveness and love" as he approaches the cross and lays himself upon it, foreshadowing his sacrifice in Chosen. To be human for Spike means to die as an immortal vampire consumed by fire -- his humanity is the fire which consumes him from within. It's not enough for Spike to simply have his soul, to suffer and burn within from guilt. The cool tones in Beneath You (from moonlight) of Spike's self-involvement and self-recrimination as he grapples with his newly restored soul will eventually give way to the warm fire of his soul in Chosen (lit by the sun) when he becomes a force for the good of others. The soul may be a source of light within, making way for conscience and guilt, but love is the source of warmth, bringing forth forgiveness and redemption.