We discussed three main aspects of costume that we were going to use in our horror opening. These were:

1. Alistair's clothing.
2. The antagonists mask.
3. The victim in the dream sequence.

Alistair's clothing

When discussing Alistair's costume, we talked about attire that a detective would wear. When researching, a lot of stereotypical Sherlock-style costumes came up. Since we wanted fairly realistic, we referenced costuming in the series True Detective. We wanted the costume to be fairly simple - dress shirt, tie, trousers, belt, badge and perhaps a blazer. I started doing drawings to explore this, and came up with these:

This is the first (terrible) concept drawing for Alistair's clothing. This is when we discussed him having a lanyard to put on in order to show his character. However, after discussion, we realised this wouldn't work very well with having a tie, so we thought he could have a clip-on ID badge instead.

We also discussed the colours used in the costume. We talked about the white shirt to symbolise him as the protagonist, but have black trousers to contrast this to represent his second, antagonist personality. We also decided on a red tie, symbolising the blood and murder of the antagonist's (or his own) victims. This could also represent danger. Visually, we will also make the red stand out in his costume by using other subdued colours for potential background characters or in the setting.

On the right is the actual design of his costume. We may change parts of the costume according to accessibility to parts of the costume, but this is what we're going to settle for. His face hasn't been drawn, due to Alistair not being cast yet.

The antagonists mask

Afterwards we discussed the design of the antagonists mask. There was a lot of speculation about it, and a lot of initial ideas included satanic appearances (for example, a goat mask with horns) and Michael Myers' type masks (simple, white). We wanted the mask to cover the entire face, that way none of his identity would be given away during the film (even though the antagonist won't be shown in the opening). In order to incorporate all of our ideas, I came up with this concept sketch of the mask, shown on the left.

We would do this by getting a typical white plastic mask, which there are plenty of online. We would then cut out the nose to create a single hole, and make the mouth wider to show the lips of the antagonist. It would also be good to incorporate goggles into the mask, which would be cut into separate eye pieces and glued to the mask. 

The victim in the dream sequence

The victim in the dream sequence didn't take a lot of work to get right. We decided that the victim should be a woman (as a representation of archetypal gender roles and fragility). We decided that she should wear white or light clothes, OR pajamas - which makes her seem vulnerable to the killer. 

Update 1

I have personally ordered the tie and the mask. An ID badge still needs to be printed and ordered, which we will need a cast photo for.