It's not scripted word for word, but it's easy to pinpoint what is planned as a storyline and what isn't these days.
The casting process is clearly long and extensive - the producers will only put housemates in there that they can work with and will provide them with good material. Certain housemates are put in there simply to cause arguments with no other storyline (Natalie this year, for example) however others are put there with a more long term view. Sam and Ryan, Jayne and Chelsie, Jason and Charley. I believe that the producer voice helps to nudge a bit of storyline progress along a bit, in broad brush strokes. Other things, like tasks, parties, rewards, secret rooms etc. are devices to ensure certain situations arise. I think, overall, the producers think of an outcome - be it a relationship between two housemates, to instigate an argument, or even in one instance a few years ago, to ensure a certain housemate wins the entire thing, and act accordingly to ensure that those things happen in a way which will be as entertaining as possible.