I found a useful video tutorial online about putting a Mist/Fog effect into a movie. The video consisted of creating 2 shape layers, adding a Turbulent Noise and masking to make a fog like background.
Here is my version of a fog effect. It looks pretty basic, I had considered confining the fog inside the walls but it would not seem natural. So it was best to leave the fog as it is. Having a figure just standing adds an eerie and sinister feeling.