I think what really sells this scene in particular is how intense Nine’s face is when he’s pondering over what Bad Wolf means and we can obviously see he’s reaching some sort of conclusion. He knows there’s something heading his way and it’s huge and it’s gotten him shaking in his boots because it may mean the end of his life. It could mean that his time is almost up and maybe he’s only living on borrowed time for every place the words Bad Wolf have appeared, death has usually come packaged with them.
And what stops him from pondering it further is when he looks over to Rose and then proceeds to act like it’s nothing. For him if he really is living on borrowed time, he wants to make sure Rose and himself are happy in his final days.