Hedgehog - Steroids can have a huge impact on mental health, yes. And coming off of them, there is bound to be an impact on mood etc because these drugs impact the brain as well as the body. Hence why body-builders can become aggressive, paranoid, suspicious when using steroids. If that mindset carries on, then yep, psychosis can develop.
Whether vulnerability to MH issues is genetic or not, no one is 100% sure. Researchers think they've isolated the schizophrenia gene for example, but even if all MH issues are genetic, it doesn't follow that all kids will develop them. There usually needs to be a combination of factors to "flick" the genetic switch.
So watching a film could be an environmental factor, if you mixed that with say a long period of stress, poor sleep, physical illness AND genes, plus the hormones of puberty, then it could seem as if the fear from a film (or Ouija board) caused it, but it's more likely to be a combination of factors. People with MH issues can often expose themselves to "dark" films, books etc, as it reflects their mood. Still doesn't mean the film caused it in isolation.
Apart from drugs, both legal and illegal, I have never known a person to develop MH issues from one activity. Ever.
Anyway, huge apologies for the derail, but felt bad not giving a full answer.