We addressed this with the character of the teenage killer. We left any background of the character a mystery to the audience - so that they might feel interested and intrigued, wanting to know more about the radical character. However, we gave the audience the information that the teenage killer was wanting to find the man in the photograph, which might be possible by tracking down his friend 'Brian Greene' - introducing the enigma of whether these characters were to survive the rest of the film, and also of course who exactly the character in the photograph actually was. There is also the obvious element of the gun prop that the teenager threatens the mother cashier with, which was also supposed to bring tension to our opening sequence.
Another way that we tried to address our audience was through the design of the antagonist. I tried many different combinations of costume with the teenager, from hooded jumpers to simple t-shirts in a bid to produce the most fearful reaction. I did this by presenting each combination (with it's own combination of make-up) to family and friends, asking if they believed my character to be a realistic killer. We settled on the design of a teenager wearing a smart dark shirt and smart trousers, with make up drawn around his eyes to make them darker and using lighting to emphasize sinister facial features.
There were also times when we received feedback from our teacher and other students in our year through our blogs on a presentation we made about our opening sequence idea. One of the key bits of constructive criticism was whether the teenager would be believable as an assassin, as most assassins would be middle aged and full of experience. We therefore left this detail out of our opening sequence and instead willed audiences towards making their own assumptions about the teenager's background. The correct assumption would have been that he was kidnapped by a cartel after his parents were killed and raised as a psychopathic killer trusted to fulfill the dirty work.
Yet another way in which we addressed a tension-hungry audience was with the character of the cashier's daughter - whose minimal role allows audiences to relate to the shop assistant and thus when the teenager draws the gun on her, makes the scene more suspenseful, as the audience would be wanting this character to survive the scene.
There was also a time when I asked our actress of the shop cashier character what she thought of the opening sequence when we had just about finalized our plans for the scene and script, she replied with:

This prompted us to leave the script be, and go ahead with our plans as they were.
One last way in which I addressed audience concerns was with the removal of one part of our opening sequence. At one point in the editing process, another group commented on how they thought that the original ending of our sequence (a scene in which another customer comes into the shop, intruding on the gunman and is left standing - visibly shocked and probably doomed to a death - before the scene cuts to black) was unnecessary and only took away from the original shock of the shooting. We addressed this by completely cutting this part of our opening sequence out during editing.