Attention to minimal details sometimes require a lot extra work only to increase the quality of a single scene but it is also a type of fanservice.
Sometimes it requires extra investigations, sometimes it requires extra time investment, and sometimes it requires extra monetary investment.
In webserials you could have it in an author going the extra mile by drawing a map for the readers or just hiring an artist to draw a character/cover/scene.
Hm.
Not sure that is the definition. Personally,
I go the extra mile of 1) drawing fanart of my characters, and 2) scrupulously creating worldbuilding for every last thing with tons of research:
Because 1 enriches my story and shows in an easier way how characters relate to each other. My story is long and complicated. I need that stuff to streamline it. And 2 is crucial to the plot and themes. There is no story without it.
Fanservice definition should be cleared up if
that's what we're talking about. Neither of the things I do is fanservice because I would have (and frequently do) the same things for the stories I do not publish anywhere -- so there are no fans to talk about. So what should I call that? My weird OCD?
What does it have to do with fanservice then?
As another example, I do not think that Tolkien had created his languages and history for decades before ever publishing it as part of fanservicing anyone either...

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So first -- define fanservice in a way that correlates with the official definitions on Wiki, Google, etc first -- because so far, I feel like you are saying that
anything that has the potential to please anybody = fanservice. And I'm not sure I agree with that.
That is just saying "anything" = fanservice and that would be nonsense. Fanservice has to mean something beyond that, some specification in its definition.
Hmm, I personally agree with OneRanter's definition though.
To me, fanservice is anything that panders to fans, sexual or not!
Then entire books and franchises can be fanservice. So it is very odd to discuss entire genres built to be nothing but serving... Like airport novels or mom porn.

.Because if it exists --> someone needs it and creates demand for it. So there's no need to ask if it's good or bad, unless you want to ask if entire genres are good or bad.
Makes me wonder how it is correlating with art in general, but then again, the first art that began was also a kind of a literal fanservice (or better to say zealot-service? Believer-service?).
This discussion then depends entirely on the hierarchy of art in question. Which art are we talking about? Is it paid art? Museum art? Referential art? Po-mo art? Naive art? Popular art? Academic art?
Because we can only determine whether fanservice is bad or good if you offer us an actual spectrum to choose from, I think...

. For some of these art movements, nude fanservice is bad while for others -- its bread and butter. Likewise, a reference to Kantian philosophy in a Pokemon is a very bad example of fanservice for the philosophy nerds, you know. Then again, I have never actually seen good philosophy fanservice in my life anywhere...

it's always just Name-Dropping and never an actual thing.
So based on what I have seen from fanservice in all areas... meh. Rather not. My preferences for what I consider "fan" usually need to go deeper than surface-level references, and the majority of fanservice NEVER go deeper than the surface.

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I hate that.
=> I hate all fanservice.