Why did Spike get brought back to the Angel series?

Question

At the end of Buffy the Vampire Slayer, we see

Spike seal the Hellmouth in a glorious blaze of fiery sun. This destroys the gaping hole, the uber vamps, and Spike.

Then,

Spike returns in the spin-off series Angel, during its last season. The medallion that he used to seal Sunnydale's Hellmouth was actually a Wolfram and Hart thing that essentially preserved his soul, and later he got his body back.

Now, this Amulet was given to Buffy by Angel, who got it from Wolfram & Hart. She in turn gave it to Spike.

Was this medallion set-up originally intended to be a way for Spike to be brought over to the Angel series? Or, did this only happen in such a way because Buffy ended?

I'm looking for something like whether or not Joss Whedon ever commented on whether he had planned all along to have Spike come back. Although, it doesn't have to be a Whedon answer, I am looking for an out-of-universe explanation by anyone involved with the show.

Answer 1

At the time, Joss Whedon stopped short of saying that it had to do with injecting some life (pardon the irony) into the dark and less well-received Angel spin-off.

We do know, however, that he was mulling whether to ship him over to Angel even before his death in Buffy. From a 2003 interview with Whedon:

WHEDON: It wasn't until the last minute that it was actually decided that he was going to be a regular on the show. When I spoke to you, a) it was true that we didn't know his status, and b) he was about to die on Buffy, so the less about him being on Angel in the press beforehand, the better.

The fact that it was seen as a ratings boost is implicit in this:

INTERVIEWER: Fair enough. But couldn't the WB hold off on announcing it?

WHEDON: There was no f---ing way. They were very excited about James. They have a shiny blond thing to show people, and they're gonna do it.

However, the medallion's link to Spike's resurrection doesn't seem to have been planned necessarily from the outset, but it could be one the "other ideas" that Whedon refers to below:

INTERVIEWER: So, how do you plan to resurrect Spike?

WHEDON: That's a conversation I'm going to be having with the Angel writers very soon.

INTERVIEWER: There's always Shanshu (the ancient prophecy introduced during Angel's first season that says once a soulful vampire fulfills his destiny, he becomes human).

WHEDON: It's not quite that simple, although a lot of people have been making reference to that. But that's an interpretation, and ultimately could become the interpretation if we decide to go that way. I have some other ideas. The trick is how to bring him back without losing the integrity of what he did... the sacrifice. If it's just, "Hey, I'm back!" then that whole moment at the end of Buffy is kind of lame now. Like Buffy returning from the dead, it's going to be something that we're going to have to earn and play the ramifications of, possibly without making it so depressing.

If anything, there seems to have been a lot of uncertainty at the time about how Spike was to be brought back to life.

(Source)