In our opening the audience are unaware that the detective is the murderer and has an identity disorder. Furthermore they are unaware that the detective is unaware that he is the murderer.
These are all plot devices that typically would be revealed later in the film, in our opening we have enforced several features to hint these plot features.
To communicate an unstable mind state we have employed the use of colour correction. The detective shots are coloured an eerie orange whilst the murderer shots are a disturbing green; both making the audience feel uneasy. The difference in colour hints at the two identities, but their disturbing and eerie qualities relate them to each other; just like how the detective and murderer are linked by being two identities in the same person.
The detective writes in a notepad in shot 9, the same paper from that notepad is used for a fake suicide note; planted at the scene of the crime, written by the murderer. The use of the same paper creates a link that they are the same person as they own the same things.
Even though the audience never see the murderer in full or even his skin for that matter, by using the same actor for the murderer and the detective, the two characters (or identities if you want to categorise them as the same character) have the same size and shape. This hint however would be largely overlooked by the fact they wear different costume. The detective is always in a suit and the murderer always has a coat with gloves. They do both however, wear the same red tie.
We could have made it more obvious through our use of shots, for example one idea would be to have the first walking tunnel shot be the detective, and the second/last walking tunnel shot be the murderer. The actor would be the same, therefore be the same size and shape , but the costume would be different; hinting at a different character. This clue would be large enough for to be a cognitively pleasure for a keen audience member but not too obvious that everyone would notice from the first watch.