Here are some posters by Channel 4 for various television shows that are not soap operas:

As you can see, there is a similar format for Channel 4 posters; the text for the title of the programme is in the bottom left often in black, which are within white blocks, and the Channel 4 logo is to the middle right. The imagery within the posters often varies and is not put to set conventions for the channel, as long as the logo and text follow the same format.
In order to make our soap opera poster conventional to the channel as well as the genre of soaps, we could follow this format. Whilst we would not be able to use the same typeface, as it is owned by Channel 4, we can use a similar typeface to make it as conventional as possible.
Here is an example of how the layout for our poster might be, without the images used:
The grey background is where our characters will be placed. Factors such as font, colors, text and sizing may later be changed to fit the images that will go behind them.