What's more important -- writing to develop the characters, or writing to develop the plot?

Most of the time I'd say the first one, because the characters drive the plot, but I've come into a dilemma.

I'm writing a fantasy that takes place over a good amount of time. There's a period of time where the bulk of the characters develop their relationships, but there's not much in the way of big plot events. If this was a shorter period of time I probably wouldn't have a problem, but it's about a year of time. What do you guys think I should do? Go straight to the plot, or go the slower route?