Sherlock Holmes: A Study in Scarlet

Sherlock Assignment: Elementary my dear Watson

1.  Sherlock, himself. He is very self-important and knowledgeable about a variety of subjects. He uses this to his advantage, not only in finding said criminals, but also in belittling the fine chaps from Scotland Yard.  This is a strong point to list as his knowledge has not come from simple study, but by being so obsessed with the intake of information as a means to an end enables him to ascertain his situation and react accordingly.
2. Watson( his PTSD to be precise)- Constantly throughout the film, there is mention of his being in the war and suffering the consequences of battle.  This in fact is not fully alluded to until the very end as we see Watson shaking thereby not having nerves of steel. Although this seems to be true, Sherlock is quick to test this by making him take chase of the cab, abandoning his walking stick and then at the end even further proving his point(unknowingly) that everything with Watson is actually psychosomatic. This holds true with the killer being shot by Watson and Sherlock saying that whoever made the kill must have had "nerves of steel".
3. Mycroft Holmes- In my opinion the red herring in this adaptation is SH's brother. From the very beginning we see he has the ability to manipulate cameras and call random payphones. With this information, we are led to believe that he may in fact, be the killer or at least have some connection to it all.  This is thrown in to keep things interesting and keep the viewer guessing.
4. The murderer keeping victim's phone.  Obviously it rouses suspicion when SH finds the victim's case(which coincidentally is pink, ie study in pink) and he notices the phone is missing.  It gives the story forward momentum and Sherlock evidence to keep pursuing the case and for the audience to delve a little deeper into analytical thought. The murderer keeps the phone as do most killers as a way to get caught. Most serial killers feel the need to show off their talents(not unlike our post-modern hero) and no one will know who they are unless they are eventually caught.
5.The killers smooth talking ability and manipulative control and Sherlock's arrogance seen throughout the film.  The whole episode revolves around people who apparently killed themselves. Then we come to the meeting of the cabbie and SH and the cabbie claims to be a genius along the ranks of the great Sherlock himself.   Sherlock outwits the cabbie and prepares to leave, as the cabbie does not have a real gun, only a replica lighter.  We now see how the killer was able to convince these people to take their own lives.  If he is able to manipulate a genius as Sherlock, he easily mentally overpowered the others. As far as the arrogance goes, SH thinks so highly of his deductive ability that he was willing to die to prove he was correct. This is why and how the killer was able to do what he did. He was good at reading people and exposing their weaknesses.