For as long as I've been writing, there's been plenty of debate among writers about what's more important: character or plot?
Clearly, the two can't be completely separated – there's no plot without characters' actions, and characters can't grow or develop without a plot.
But many stories will come down more heavily on one side than the other. In thrillers or adventure stories, for instance, the plot is generally the most important thing – the reader wants to find out what happens next, or wants to learn the solution to a particular mystery.
In, say, romance novels, there might not be much action – but the story is driven onwards by the "will they or won't they?" tension between characters.
Readers may well come down on one side or the other in terms of what they prefer.
Personally, I'm drawn to characters. I get a bit lost in books (and TV series!) with fiendishly complex plots – but I love characters who are interesting, flawed, complex people. When I read more plot-centric genres, I tend to gravitate towards the more character-rich end of those (e.g. with thrillers, I read mostly psychological and domestic thrillers, by authors
like Sophie Hannah and Liane Moriarty).
When it comes to writing, though, you can't simply let your characters interact and hope that a plot somehow emerges ... and on the flip side, you can't create a plot that doesn't take into account your characters' strengths, weaknesses, and motivations.
If your characters are going to grow and change in some way during the course of your novel, that needs to be both a result of events and a cause of them. For instance, if your protagonist's failure in Chapter 5 leads to a lot of soul-searching, their newfound willingness to accept help might well lead to their victory in Chapter 30.
It's easy for character-driven writers (like me) to get focused on enjoying writing about their characters without having enough happening in the story (or without having it happen at the right pace).
If you're more of a plot-driven writer, then it's easy to end up with a situation where your characters aren't behaving in believable ways -- because your plot requires them to do something unduly reckless or stupid. (We're going to cover how to fix situations like this later in this series when we tackle plot holes.)
When you're planning and outlining your novel, I think it doesn't matter too much where you start. I tend to start with a character facing a key problem or situation, which I think is where many writers begin: at the intersection of plot and character. But if you have a vivid character and no plot, or a clever plot idea but no characters, that's a perfectly fine place to start too.
Ultimately, your characters and your plot are closely joined, and you could even see them as different ways of looking at the same thing -- a bit like the way light is both a wave and a stream of particles.
While you might well have a story that's more focused on characters than plot (or vice versa), you need to make sure that both are working seamlessly together in order for your story to be as good as it can be.