How legitimate is to improvise?

As the Keeper, I must say what my preparation demands; however, do I have room for improvisation and if so, how much?
During our last session, one of the characters goes to the hospital to get some bruises fixed. The EMT brings in a girl; she would be gorgeous, if half of her face wasn't crushed. The character goes out, talks with the ambulance driver and discovers that the girl has been brutally beaten by her father (I'm injecting horror into everyday situations). The players decide to investigate the fact - a completely casual even that I threw in because I felt there wasn't much horror in our sessions.
And then a thought comes into my mind.
Yes, of course Laura (the girl) is important. But not because of her father, no. In fact, he's not her real father. She's the daughter of this mistery's monster, a ghoul. Her foster father discovered her eating a human hand, freaked out and almost killed her, then stopped and called the ambulace because, after all, he just almost killed his daughter (he had some hints she was not his biological daughter, but still). Yeah, she's a monster; but again, yeah, he broke a 16-year-old's jawbone. In more than one point.
All this was improvised; it wasn't in my preparation at all. Did I do something wrong by introducing Laura and her father?

No, of course you didn't do that wrong.

"Say what your preparation demands" doesn't preclude improvisation. Instead, it keeps your prepared mystery at the centre of things.

So, adding a new bystander and tying her to the (prepared) mystery in a way you hadn't thought of is fine. It's in line with your Keeper principles, and you have to say what the principles demand as well.

If you had decided instead that her father was an alien and there wasn't a ghoul after all (making things that the hunters had already discovered now untrue), that would be bad.

I guess the key thing is that improvisation should add to the game, rather than change what's already established.

I guess the key thing is that improvisation should add to the game, rather than change what's already established.

That's great. :-) Thanks!