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In my novel, at the point where the MC meets her would-be boyfriend, Rufus, I have added a small backstory about his job. Later on, the story is almost entirely focused on the MC and what happens to her with a very little mention of Rufus' backstory.

Now, IMHO, being a lead character, his backstory is something the reader would like. Three chapters after meeting Rufus they fall in love, and then the story is almost focused on the adventure they go on together from the MC's POV.

I am unable to understand, which characters need a backstory, wherein the story it should be introduced. Are backstories 'word fillers'?

A backstory needs to matter to the story; in this case it probably matters to the MC; few people fall in love with a person they know nothing about. They fall in lust, certainly, and that just can lead to true love by compelling them to pursue the object of their lust and thus get to know them personally. But lust isn't love -- If in getting to know the object of their lust they find repellent ideas, cruelty and selfishness, disgust with the personality can defeat both love and lust.

In that sense, you may want the Rufus backstory mostly related TO the MC, to create sympathies and understanding for Rufus with her, supporting the cause of her falling in love with Rufus. It becomes more convincing to the reader if they share in the MC's experience of Rufus, instead of them being told something the MC doesn't really know. And story wise, this is more efficient: If you tell the reader something the MC doesn't know, and then later Rufus needs to tell her all about it, then as an author you are repeating yourself and boring the reader, because when they read this second passage, the reader is not learning anything new at all.

There is some magic in romantic love, some undefinable qualities are necessary, but there are also some commonalities we expect to be present for true romantic love, as opposed to similar motivators like lust, or chivalrous honor, or heroism in protecting the weak. One of these commonalities is spending time together and getting to know each other, personality is an important component of love, but not so much the similar motivators listed; we can lust after a beautiful body without knowing a thing about the person inside it. Or pull such a person out of a car crash, or intervene to prevent a bully from beating them.

Romance demands a fit of two personalities, usually one with synergy, so the two together are "better" in some sense to each of them, than they are alone. That synergy is the magical part that you, as an author, can assert as feeling.

But if you want them to fall in love, they do need to get to know each other, even if much of that is off-screen, you have to make sure the reader understands they are spending time together and learning about each other. Wherever you engineer your story for these conversations to take place, that is a good time for backstories.

These "times for conversation" often occur in transitional times within the story; for example while traveling somewhere, or on a stake out, or in an office setting working on a project, or while one of them is in the hospital recovering from injury, etc. Avoid a "talking heads" scene, make it so they are doing something (or things are going on around them) that doesn't require a lot of mental attention and allows them time to have a conversation.

First and foremost, every character requires a backstory in your mind. You need to know who they are, why they act in a certain way, how they would respond to new situations, etc.

Once you have that backstory, you can decide how much of it will be revealed to the reader, at what point of the story, and whether directly or indirectly. Bits and pieces of a character's backstory can be strewn throughout your story, coming up when they're relevant. It might be that some element would be revealed when this would create the most dramatic effect. Or it might be that the backstory remains entirely in the subtext, never getting officially "revealed".

Look for example at Hemingway's The Sun Also Rises: the novel starts with the backstory of one character, Robert Cohn. The backstory of another, Brett, is revealed in bits and pieces - here we learn that she's getting married, there - that she's getting divorced first, much much later - why she's getting divorced. As for the MC, we learn very early on that he suffers from impotency, or something similar, due to an injury suffered during WW1. We never learn any more. His backstory is all subtext and no text.

Or, look at The Lord of the Rings. Boromir tells us his backstory right upon arrival: who he is, where he came from, why, his family, his position in society - everything. Aragorn, on the other hand - it takes several chapters between his appearance as "Strider the Ranger" and the revelation that he is Isildur's heir. Of Legolas's and Gimli's backstory we know little save where they came from, since that's irrelevant; and Gandalf's backstory is a deliberate mystery. But Gandalf's story does exist, it just never gets told. You find out more about him in the Unfinished Tales.

Ultimately audiences don't need a lot of details about most characters' history, what they do need, in order for the story to make sense and be immersive, is justifications for their actions. If a character has clear (stated) motives, reasons for being in the tale and doing what they do that is enough. You as the author will generally know far more than you ever put on the page about where characters come from and their true motives. Rufus needs a reason to attach himself to the MC and the two of them need reasons to bond, backstory may form the basis for this, but equally the events of the narrative may be sufficient without a detailed history of the characters coming into it.

Note that stated motives and true motives need not be the same thing, the friction between the two can be a source of dramatic tension.