I was on the bus...

[Donovan]

And in this case the vocal interruption does two things: it makes the potential perv know people are watching and draws attention to the unfolding situation (possibly defusing any other nefarious intent) and lets the girls, who are probably creeped out and a little nervous, know that people are aware of what's going on. The guy was past the creeper line when he got up and moved to get a better look.