Q1: How similar are Buffy and Spike, personality-wise?
That's a question that volumes could be written about. From the very first moment that we saw Spike, he was always unique among vampires. More powerful/dangerous. He had killed, single-handedly, not one, but two Slayers, and had no trepidation about making Buffy number three. Where most, if not all, other vampires seemed to fear the Slayers, Spike sought them out, challenged them one on one, and apparently fought quite honorably when he did find them.
Like Spike, Buffy was unique in her loyalty to her friends (as far as we know, she was the only Slayer to actually have friends who fought beside her). Also, she was, or at least was presented as being, a much better and more powerful fighter than her predecessors. Whereas most Potential Slayers are trained from childhood, she was literally handed a stake for the first time when she was Called. With no training at all, she managed to beat, not one, but what was apparently a good sized group of vampires back in LA before moving to Sunnydale. And though cautious about it, Buffy never seemed hesitant to engage in fights where she appeared to be under matched, but she always managed to come out alive.
In the end, Buffy and Spike are both heroes and champions who defied all the odds and refused to fit into the molds that should have defined them, and the major thing that actually made both unique was their ability to love, their loyalty to those who they did love, and their unswerving commitment to what they ultimately considered to be their mission. Both were warriors at the top of their class, but both had a gentler side as well. Both were at the top of the "People Who I Most Definitely Do NOT Want Pi**ed Off At Me," but at the same time they were also at the top of the "People Who I Want On My Side When The S**t Hits The Fan" list. And, ultimately, both unhesitatingly gave their lives to protect the people who they loved.
Q2: Is there anything you'd change about the handling of Spike and Buffy's relationship in seasons 5-8?
I would have had their relationship in Season 6 to have remained as gentle and actually caring as it was toward the end of Season 5 and the beginning of Season 6. The man who swore to defend Dawn "til the end of the world," and who saved his Slayer every night for 47 nights just wasn't the same man who told Buffy that she "came back wrong" or tried ti convince her that she belonged "in the darkness...with me." The girl who was grateful to an erstwhile mortal enemy for suffering torture rather than betray her and her family, who counted unquestioningly on that same former mortal enemy to take care of her sister just wasn't the same one who later saw him as a soulless thing, capable of selling demon eggs to terrorists, but too stupid to keep those eggs from hatching prematurely.
In Season 7, would have had them both get past the bad things of the year before and quit pretending that they both felt a tremendous attraction for the other, an attraction that with just a little encouragement could have grown into a love of truly epic proportions.
In Season 5 of "Angel," I would have had Spike keep his resolve to find Buffy as soon as he was able to do so, and conversely, I would have had Buffy much more determined to seek Spike out once she became aware that he had returned.
By Season 8, I would have had Buffy and Spike as a couple recognized and accepted by everyone in their circle of friends and allies, and if Angel or Giles didn't like it, they could have shoved something large, rough, and very pointy as far up into whichever of their orifices that would be the least comfortable and the most damaging.
Q3: In terms of your interest in Spike/Buffy, which was your lowest point in the series? Your highest?
Certainly the events of "Seeing Red" would rank right down there with the lowest, but, for me, the actual lowest point in the Spuffy relationship was there in the basement in "Wrecked" when Spike said, "I always knew that the only thing better than killing a Slayer would be..." We'll never know for sure, but I've always believed that that simple incomplete sentence set Spuffy back months if not a full year.
The highest would be "Touched," in particular Spike's "You're the one speech".
Q4: And lastly, a comic-focused question, When it comes to Spike/Buffy in the final issues of season 8 , what is...
*Your worst nightmare?
That Spike and Buffy will not be given the time and opportunity to simply sit down together someplace private and finally, honestly, and fully, analyze their feelings for each other and decide whether they will ultimately be friends, lovers, or simply allies when the need arises. We've hemmed and hawed around for way too long, dancing around mixed signals and lost opportunities. It's way past time to resolve this, whatever the ultimate answer winds up being. For what he's suffered, for what he's accomplished, and for the Champion that he has become, Spike deserves more consideration that that.
*Your greatest fantasy?
Buffy, Spike, Faith, and Willow running a Slayer academy somewhere secluded and scenic, where the rest of the world doesn't keep intruding in on their well earned peace and quiet (Don't ask me how Faith and Willow got into the fantasy, the four just seemed to feel right together)
*What you think will happen?
I think that Season 8 will end with Angel and possibly Dawn dying and everyone else left in a sense of shock over how it actually happened, which will lead into season 9. Season 9 will have a very strong sub plot involving the quest to find a way to bring one or both of them back, ultimately resulting in Angel's Shanshu award finally coming about. As for what that will mean for Spike and Buffy, Joss has said repeatedly that Buffy and Angel ultimately belong together, so ultimately that's probably what's going to happen, regardless of what we might believe.