Jerome Stern writes in Making Shapely Fiction that even though tension underlies suspense through all of the tools you would suspect . . . pacing, plot, hints of hope and fears, "the center of real suspense is character. Readers have to be emotionally involved before they can suffer your character's disappointments . . .."
Donald Maass tells us in Writing the Breakout Novel "we cannot help but like people that we know very well, whatever their faults."
My mentor, multi-award winning romantic suspense author, Colleen Coble, takes the ideas of emotional involvement and knowing your main character a step further. She says a reader must be able to identify VERY CLOSELY with that person. A slight nuance in character development maybe, but an important concept to consider.
Think about it. A suspense novelist can create a protagonist a reader cares about because they feel they know her, but making her so identifiable almost anyone can imagine themselves in her shoes--that's truly transporting a reader using strong characterization.
It's all better with friends.