One of the suggestions I got on commentary topics was a look at Spike and Buffy's relationship before the reveal. How could he be around her, pretending to be innocent, helping her with her investigation, even suggesting other suspects, knowing what he'd done?
Well, short answer: he's evil. Morality isn't quite as much of a hurdle as it would be for most people. The long answer is under the cut. :)
Their first post-sex interaction comes in Chapter 2:
"What the hell did you do to me?"
"Don't know what you're talking about, Slayer," he protested as she pinned him in place with one strong hand on his chest.
"The party last night."
Something flashed across Spike's eyes at the mention of the party, but it was gone in an instant, and Buffy questioned whether she'd just imagined it.
"What about it?" Spike asked cautiously.
"Someone raped me."
His face flickered with a myriad of emotions, finally settling on shock and indignation. "And you think it was me?"
Notice she never directly asks the question, and he never directly answers it. A few astute readers picked up that his facial expressions give away that there's something going on under the surface. Basically, I felt that Spike would be anticipating an angry Buffy (remember his hesitation during the sex itself, worrying she'd stake him for it), so he wants to hear her interpretation first, and he's not going to admit to anything she didn't ask. He wasn't expecting the memory loss, though, and once he realizes she thinks she was raped, he's really not going to say anything, what with Buffy in a "stake first, ask questions later" kind of mood. At this point, he does feel a little bit bad that Buffy's upset - enough to comfort her, but not enough to tell her the truth. His selfishness and self-preservation win out on that one.
The next time we see them together is in Chapter 3. Unlike most of their later interactions on patrol, this first time Spike does NOT intervene to help Buffy - he intervenes to put the poor mutilated vampire out of his misery. Spike's not quite at the "helping Buffy out" stage yet. But he does show some sympathy:
"Buffy," Spike said quietly. "Are you all right?"
Their interaction here is meant to show him softening up a little. His caretaker instinct is already starting to creep in, although he's still trying to fight it. They're mortal enemies, sure, but they had a connection that night, and now she's suffering for it. In other words, Spike, meet guilt. You're going to become very good friends.
We next check in with our couple in Chapter 5, again in the graveyard. This time Spike DOES save Buffy, and despite his protestations, it's because he's started to care about her. He can't help feeling for her, because he knows what it's like to feel helpless, to be reduced to a victim. But he's still warring with himself, and trying not to dwell on his own role in making her feel that way:
"But you wouldn't have minded if it… Christ, Slayer, is this 'cause of that rape business? You need to get yourself some help."
Now that you know he's partly responsible, doesn't that seem like a terribly insensitive thing for him to say? But this scene is about him trying not to care, trying to distance himself from her and her situation, because he knows he's starting to fall for her, and that scares the shit out of him.
But no matter how he tries to avoid it, he always ends up helping her, whether it's talking to her about nothing in Chapter 6, or welcoming her into his crypt in Chapter 7. But he's completely unprepared for the conversation she wants to have:
"My counselor thinks it'll help me if I try to find out who raped me."
"Oh, yeah?" he said, a nervous tone in his voice, fidgeting in his seat.
On first read, most people probably agreed with Buffy in attributing Spike's reaction to him being nervous because she's confiding in him. But really, it's because she just said she wants to find out who raped her, and he's thinking he's gonna get outed.
"I feel like I can talk to you… like I can trust you."
She risked a glance up, and she thought she saw the vaguest hint of discomfort at the notion, but it was mostly drowned out by the sympathy in his eyes.
Spike's discomfort obviously comes from knowing that she shouldn't trust him, but he can't keep the sympathy from overwhelming him.
"Wait." Spike held up his hand. "You don't think Harris, maybe...?"
Spike is deliberately sending Buffy off in the wrong direction here. And yet, he's terrible at it, choosing the person she's least likely to suspect, and most likely to be angry he would suggest it.
He whirled around to face her with an apologetic look. "Buffy, there's - there's something I have to tell you. Should've said something before, but…" He shook his head.
He cares for her now, too much to keep on denying it (not to mention feeling guilty that she's so trusting of him), and he's SO close to confessing, but he chickens out and misdirects again by turning the conversation back to Parker. You know, now that I think about it, perhaps the reason Buffy's investigation never got very far was because Spike kept encouraging her to pursue dead ends...
One thing I should note - Spike's almost-confession was added well after I'd written the rest of this scene. As I've probably mentioned in the comments, I was writing this fic several chapters ahead of what I was posting, which came in handy when I wanted to go back and revise. I think I'd posted maybe up to Chapter 6 (but had written up to Chapter 11) when I revealed to
slaymesoftly my evil plans for Spike. And she went, "You wanna WHAT??" (I may have paraphrased that a bit.) She thought Spike's involvement would come as too much of a shock because he hadn't shown enough guilt or conflict about it to hint that he might not have been totally honest with her. So I added this bit to the scene, as well as another hint of Spike's conflicting emotions in Chapter 8:
He knew he didn't deserve this, didn't deserve her, but each passing day made it that much more difficult to face the truth. He couldn't risk losing her now.
By now, Spike has passed the point of no return. He may not be quite in love with her yet, but he's obviously fallen hard, and he can't bring himself to come clean, because he knows she'll be upset and he'll lose her. So he just keeps digging himself in deeper and deeper - the longer he goes without telling her, the angrier she'll be at him, and the less likely she is to forgive him. But the bigger the risk of losing her, the more determined he is that she never find out, despite the guilt that he feels. I know some people have wondered how Spike could possibly love her and yet not tell her the truth, but I think this situation is probably familiar to anyone who's ever been caught in a lie like this - you just keep going because you know it's going to be awful when the truth comes out, and you want the happiness to last as long as you can. (It's also probably familiar to anyone who's ever watched a romantic comedy, lol. They're always making movies where couples get together under false pretenses - one of them's lying to the other about who they really are, or there's a misunderstanding that no one bothers to correct, or they get together because of a bet or an assignment, and then the other person finds out and gets pissed. But it always ends with them making up!)
Everything after this point is just Spike getting deeper and deeper into his feelings for Buffy, feeling guilty about the secret, but unable to resist the promise of a relationship with her. His guilt really shows in Chapter 15, after they've had sex for the first time (and also to build suspense toward the reveal of Spike's role). A few examples:
Spike ran a hand lightly down her bare arm. "Were you dreaming about…?" She nodded, and his hand paused, hovering at her elbow. "Did you - remember anything? Could you see who it - who it was?"
When she confirms she was dreaming about the rape, he freezes, and his halting questions are trying to get at, "Do you know it was me?" Spike doesn't know at this point that there was someone else who really did rape her, and so he thinks Buffy's reaction is subconsciously directed at him.
Spike breathed a sigh of relief. He'd been terribly afraid that she would think it was all a mistake in the morning, that he'd ruined things by reminding her too much of what had happened, or by pushing her too fast. Having her wake up screaming hadn't exactly eased his fears any. His heart had leapt into his throat, so sure that she would wake up and accuse him, that something about last night must have triggered her memory and she would think -
And she would put together two and two and realize it had been him. For Spike, this is even more nerve-wracking than one might expect on first reading. He's playing with fire here, just by having sex with her - the very act could've brought back her memory, revealing the truth to her.
One of the things I relied on a lot in building their relationship was the sense memory - somewhere inside, Buffy DOES remember sleeping with Spike, and she remembers enjoying it. She attributes a lot of her comfort to the "Something Blue" spell, but I think there's some of the other experience mixed in as well. Originally, the clues were much more heavy-handed - in this chapter, I actually had Buffy talk about a sense of déjà vu, that it felt like they'd done all this before - but I decided that was a little too obvious. (I also had a joke about Angel and first times, which would also have hinted that this wasn't Buffy and Spike's first time together, but that was way overkill.)
Anyway, back to Spike's guilt:
He hadn't known when she'd first burst into his crypt, all righteous anger and accusations, how hard he would fall for her. He'd thought it would be easy, tell her what she wanted to hear, get her to forget this whole mess. He hadn't counted on wanting to help her through it, wanting to help her solve the mystery, even as he withheld the missing piece. He should've come clean, but it was too late now; he was in too deep, and he couldn't risk losing her. His desire for her had overwhelmed his judgment, his self-preservation, even his guilt.
As you can see, I didn't need those other clues, because this one was a big flashing neon sign that said, "Spike's hiding something." Not everyone jumped to the right conclusion - some people thought he just had information on the rapist, or knew about the tape - but there was no denying that Spike had a secret that was eating at him, that he wanted to tell Buffy about, but couldn't without possibly losing her.
I think, in retrospect, Spike was pretty much on the nose about Buffy's reactions. She admits that she would've staked him right away if he'd told her way back in Chapter 2, and her anger after she finds out is exactly what Spike was afraid of. He ends up having to endure a month of not being with her - and he's lucky that she managed to watch the tape of them, otherwise who knows if she'd ever have been able to forgive him. So, while there's no question that what Spike did was wrong, to me at least it makes sense why he kept it a secret for so long. I think that's a theme for a lot of the characters in this story - they may not do the right thing, but it's understandable why they do what they do. I'd put Riley into that category, as well as Willow and Xander, and even Graham to some extent.