I’m afraid I’ll have to call ‘no comment’ on that one. I only sporadically follow news and interviews about the actors and the show, and so I have no idea where that particular fact came from or the context surrounding it. Since I’ve heard so many people quoting it as a fact, I assume it isn’t just word of mouth, but I haven’t hunted it up to find out for myself.
If they did keep it a secret, then I think it was probably in order to affect the performance. I’m pretty sure she’s been privy to other secrets they didn’t want to get out, so I doubt it was a matter of trust.