During the first three seasons of Fringe, the Observers are mainly just there, observing. It slowly transpires that some of them have played a more active role.
Then, suddenly, starting with S4E19 and continued in S5,
they're invaders. The Observers we saw in the first three seasons, were just a science team, exploring.
Was that planned from the start? Or was it just a possibility created by the writers?
As Pinkner said in an interview:
Pinkner: Oh, yeah. We purposefully plant seeds along the way and sometimes we know exactly how they're going to pay off. Other times we know that they will pay off, but not necessarily when or how. And we've said we sort of want to world-build. It's a way to make the show feel like it takes place in a world and it's not just episodic.
So was this just the outcome of the planted seeds, or was this something that had been planned from the start?