When I mentioned that I was going to write a Spike/Lilah essay, one of my friends asked, "How do you go about writing an article shipping two characters who've never met?"
Good question. Better question, how do you go about writing a story shipping two characters who've never met?
Okay, so maybe it's a slightly more daunting task than working with an established canon couple, or people who at least interact on a regular basis, but half the fun of fanfic is finding creative ways to re-shape established canon to your liking.
Technicalities
Getting Spike and Lilah in the same room, luckily, is actually pretty easy. Easier than Willow/Kate Lockley shippers would have it, at any rate. Lilah works for Wolfram and Hart and Spike's a decently-connected vampire. There's plenty of reasons for Spike to seek the firm's help, and vica versa. The Senior Partners want to recruit Spike, Spike needs something the only firm can locate for some wicked scheme, and so on. There's also plenty of room for fic set at the firm post-Chosen/Home with barely-back-from-the-dead Spike and Lilah working together in Wolfram, Hart, and Angel.
We know Spike was in Los Angeles early in Season 1 ("Harsh Light of Day"/"In the Dark"). We also know he didn't necessarily head straight back to Sunnydale, as there was a two-episode lull, "Fear Itself" and "Beer Bad," before we saw him get captured by the Initiative in "Wild at Heart." We also really have no specific idea what Spike and Lilah were doing between seasons 4/1 and 5/2. Those are just a few basic ideas. Really, the sky is the limit when it comes to getting these two close enough to let the snark and sparks fly.
Birds of a Feather
There's no law that says every pairing has to come down to sex. There's plenty of room for friendship fics, and oh, how Spike and Lilah could get along well on a friendship level. There's that whole "my White Hat lover dumped my ass because I'm too evil" ("As You Were," "Habeas Corpses"), the really complicated relationship with Angel, the general willingness to let people know what they really think of them, and good taste in liquor.
Both Spike and Lilah seem to be well-read. We know Spike was a poet as a human, and we've seen him reading since ("As You Were"). Lilah knows her Dante ("A New World"), and many lawyers start out as English majors in college. Both have a decent knowledge of pop culture, and you just know Lilah probably watches Passions though she'd sooner give an erotic massage to Gavin Park than admit it publicly.
In terms of "evil," Lilah and Season 2 - 4 pre-In-Love-With-Buffy Spike seem to have the same stance. While either one would gladly sacrifice some newborn babies to save their own skin, neither have shown themselves to be too into sadistic torture like Angelus and Drusilla. Exceptions on both parts, of course, made for seeing Angel tortured, but that's more a personal vendetta than anything else.
Yes, I am aware of the torture-victims-with-railroad-spikes deal, what Spike said in "Never Leave Me": "Do you know how much blood you can drink from a girl before she'll die? I do. You see, the trick is to drink just enough to know how to damage them just enough so that they'll still cry when you-'cause it's not worth it if they don't cry." Personally, that sounds a lot like Angelus and Drusilla's influence over Spike, and while he probably enjoyed it, I'm thinking it probably wasn't his idea originally, and more about the reinvention of William. Besides which, it would take way too long to properly explore Spike's morality, and better essayists than me have already thoroughly covered all that anyway.
The point is, Lilah and Spike are generally more concerned with making themselves happy than going out and ending the world. Both have worked with the good guys when it was in their best interest ("Becoming 2," "Apocalypse, Nowish"), and both might have joined the White Hats full-time if they seriously thought Good was gonna be the winning team.
Sealing the Deal
So there's plenty of things available to bring Spike and Lilah together as friends. Now say you want to take it to the next level. Here's where their never having met can actually work to your advantage. Why? No baggage. Spike has never met Wesley, Lilah has never met Buffy. As far as we know from canon, they've never slept with the same people. Lilah and Spike have not tried to kill or corrupt each other, Wolfram and Hart has not tried to harm Dawn or Buffy or anyone else Spike cares about, and Spike never chose Angel over Lilah. There are no weird pre-existing problems to work through before you can pair them up. Plus, Lilah doesn't seem to have the black and white "vampires are bad, very very bad" outlook that most Buffyversers have.
Spike is, of course, love's bitch. If you're even reading this far, you probably have a good idea of Buffyverse canon and the characters. If you really want to know more about Spike's loves, head over to the Spike section of this site, or check out two fabulous essays by Jessica Walker, Love's Bitch and But vampires can't love... right?. But in a nutshell, when Spike loves, really loves, a woman, it's a deep, devoted, obsessive, "eternal...literally" deal. This means that you're likely going to have to deal with The Buffy Factor if you set your fic after mid-Season 5, and explain why Spike is with someone who isn't Buffy. The Mutant Enemy writers found a good bottle of whisky to be the easiest way of getting around that ("Entropy").
Lilah, on the other hand, has not been shown to be a serial monogamist like Spike. The truth is, Wesley aside, we don't canonically have much else to go on when it comes to hard facts about Lilah's lovelife. There was, of course, the slashy tone in her scene with Faith in "Five by Five," and the fact that she thought it was Angel, not Marcus, she was kissing in "Carpe Noctem." But the only person we know her to have had an on-going relationship with was Wesley. And even that seemed to take her by surprise; she actually looks hurt when Wesley tells her, "I wasn't thinking about you when you were here," despite the fact that she's just finished reminding him that she's not "one of the doe-y eyed girls of Angel Investigations. Don't be thinking about me when I'm gone" ("Tomorrow"). All that being said, if you're setting your fic post-Habeas Corpses, you're probably going to have to deal with The Wesley Factor, because they did show that she loved him.
What does all this mean when it comes to actually putting Spike and Lilah together? It really depends heavily on what time period you're setting your fic in, as both characters went through a lot of changes in later seasons. Within canon, it's very hard to plausibly bring Spike and Lilah together while Spike is still with Drusilla. If you're going the Alternate Universe route, all you need to know is that Lilah and Spike are very flirtatious, sensual people who are concerned with their own comfort and level of amusement. Season 4/1 also lends itself to casual sex, but you don't have to rule out a True Love story if you want. After all, Spike was over Drusilla by that time, and while Lilah probably hadn't allowed herself to fall in love often if ever, we can see from her relationship with Wesley that she's certainly capable of falling in love.
Seasons 5/2 and 6/3 are very much tied to The Buffy Factor for Spike, and 7/4 The Wesley Factor for Lilah. Again, unless you're doing an AU, your best bet is to acknowledge their other relationships before exploring their relationship together. Relationships like Spike had with Buffy and Lilah with Wesley can be very draining, very all-encompassing, emotional and exhausting. After all that drama-rama, they might like nothing better than to pass a few hours with someone who's fun, sexy, and understands them. And if you're on-board with the Wesley Unification Theory, which says that Wesley "is a lot more similar to Dru than most people realize" (Section VI, M), then you know this would make Lilah the Spike to Wesley's Drusilla. (Try saying that three times fast!) It might be a welcome change for both of them to indulge in a little debauchery after spending so much time worrying about their lovers.
Miscellaneous
Spike and Lilah don't really lend themselves to fluff-fic. Lilah is never going to wake up in Spike's arms and go "Who's my widdle cuddle-bear?" At least not without heavy sarcasm and chuckling. That doesn't necessarily mean you have to make a fic with them dark and gloomy. Spike and Lilah get the lion's share of funny lines and scenes. Horrible, horrible jokes about the price of getting blood out of carpeting, and little girls in coal bins. Neither characters take the world on a whole very seriously, and they're quite willing to make jokes at anyone's expense, except maybe their own (even though the soul softened him up a little, Spike remained as proud as Lilah to the end).
Spike and Lilah are a very easy couple to bring BDSM themes into the bedroom with. First, hello, Spike and Drusilla. There's also evidence to suggest that Spike/Buffy and Lilah/Wesley had their own boughts of rough, kinky play. As far as who's on top, it's probably safe to say your call. On the one hand, Drusilla enjoyed torturing Angel in "What's My Line? 2," but Spike seemed to think that tying up and torturing Drusilla was the way to win her back in "Lovers Walk." So he was probably on the receiving end in that relationship just as often as he was the one doling it out. We have no evidence of Lilah's preferences, but control freaks tend to either bring that into the bedroom, or they choose to take a break and hand the control over to their partner. So really, it can go either way with them.
Writing a Spike and Lilah story may take a little extra work and creativity, but you are given more liberty and freedom than some established pairings, and the reward is getting two pretty, snarky, morally ambiguous, decadent people together. And there is no bad there.