I was playing in a band and was approached to score an independent film. I had never done it, but had written instrumental music, so I figured I could do it. Turns out I loved scoring the film, and took on another couple films before realizing that if I was to be an effective narrative composer, I should study the craft of composition. I stopped taking projects and got a degree in orchestral music composition, and followed that with film scoring studies. Near the end of my degree studies, I started taking on student films as a way to get back into film scoring.
I've been fortunate in that the films I've worked on in the horror genre are themselves not pure horror, and have allowed me to write in a wide variety of styles. Those scores contain elements of fantasy, drama, action, comedy... really all types of scoring, and that gives the horror moments more impact. As for scoring the horror moments, I do like approaching the music from the psychological aspect, scoring to the characters' thoughts, emotions, motivations and such.
I use all types of instruments, really depending on the film. Instrument choices are very much tied to lighting, colors, art direction, as well as the narrative elements. I have a great collection of vintage synths, and of course I do like to write for acoustic instruments. I find the depth and intricacy of sound and emotion you can get with acoustic ensembles extremely versatile and effective in the overall sound environment. Also, the human aspect of performance is such an important part of the music score to character connection.