Yeah, that can be tricky. I remember an episode of MONK where Stottlemeyer's previously anonymous secretary got enough screen time in the opening scenes that you just knew
that she was going to end up being the killer somehow.
That being said, I admit my first response was "Hastings? Who the heck is Hastings?"
If you're going to pull that trick, you need a character that will at least make some
impression early, so viewers won't be scratching their heads at the big reveal.