One question that comes up pretty often among novelists is "What's more important – plot or character?"
Often, this crops up in the planning stage as "which should I think about first?"
If you've been writing fiction for a while, you'll probably already know that the "right" answer to this question is to say that both plot and character are equally important, and that they can't be separated from one another.
Characters are instrumental in creating the plot; the plot affects who those characters are (or reveals who they really were all along).
I think that's fine as far as it goes ... but realistically, for most writers, either plot or character will get more emphasis.
For instance, if you write thrillers, they pretty much have to be plot-driven. Even psychological thrillers have a strong emphasis on plot (often through revelation of back story, through twists and surprises).
Literary novels, on the other hand, tend to be quite strongly character-driven. There may not be a lot of action, but there's plenty of exploration of characters' internal worlds – or of the relationships between characters.
Know what's expected from your genre. Make sure you match up to that – and, if you can, try to exceed expectations. For instance, you might write a thriller that has a conventionally strong plot but also compelling, memorable characters (think Hannibal Lector in Silence of the Lambs).
In general, I feel that characters are what readers remember. If you want to keep them coming back for more, you will need deep, interesting characters who grow and change.
But a weak or slow plot can lose those readers before they ever get to really know your characters ... so that matters too.
Recommended Read:
If you're struggling with either plot or character – or the intersection of the two! – then K.M. Weiland's book Structuring Your Novel does a great job of bringing the two together, demonstrating how great plotting and great character arcs work hand-in-hand.