There are means for conditioning to encountering the otherworldly, but that is mostly touch and go type events. Exposing one's self to the unnatural in small, controlled amounts that are not traumatizing will, undoubtedly with time, make it easier to accept or witness greater things. So those characters with a bit of occult experience are more mentally hardened to it, just as a person accustomed to confronting fear or violence. Short of "natural" exposure, unnatural means such as some wizard enchantments can help dampen the mental shock.

The cast should be made up of people with credible backgrounds and history. You are perfectly welcome to portray the spectrum of humanity, but the only limit being that they must be of age at minimum; no child characters.