How do you roleplay a first exposure to killing?
As an acting technique, you have three major choices.
Go with the obvious- Loud: Freak out, run around, wail, wave your arms, cry, scream, etc. You've just be exposed to a horror beyond horrors: death. It's grip over your own mortality is frightening and disparaging. It twists at your very soul and charges every fiber of your being with an uncontrollable anxiety the likes of which you have never before experienced. You show fear like a man possessed. (This is most likely the healthiest psychological response) Major target emotions: Sadness, Fear.
You go with the opposite: Quiet. While physically you may be covered in blood and guts, emotionally, you are more withdrawn than outgoing. The shock of rampant death and destruction has shut off your personality temporarily. Your dreams may be flooded with screams of the dying, but in your waking life you are often mistaken as the strong silent type. In reality, you are now the silent and tortured type who has to be strong to never experience that again. You are stressed, and know exactly why. Seeing representations of violence, weapons, etc set you on edge. You may make emotional explosions at or about these things often. (Psychologically, this is the mid-range response, you can recover, but it'll take months, if not years) Major Target Emotions: Stressed, Angry
You go with the denial- Forget: "Nobody died. This is just food stains. I'm a really sloppy eater! Haha, yeah..." Your mind blanked out the entire experience, and will continue to do so every time you experience a similar event. You lie to yourself constantly to continue to hide the truth from yourself. You can continue to function in combats as the scent of blood brings back the traumatic memories, but once the danger's passed, you black out and come to with no recollection. (Psychologically, this is the more severe condition, but it may clear up quicker as you adjust). Major Target Emotions: Happy, Casual.
If your character is average, start with 2. (A PTSD-like response) If they're strong willed, start with 1 (They freak, but do not lose their minds). If their will is abysmal, start with 3 (This is a psychotic break from reality).
How long would it take to get an otherwise normal person to get used to the way your average adventuring party behaves?
Recovery from the experience of death is not measured in time but instead with assistance. Desensitization (which has to happen BEFOREHAND) can be accomplished in a matter of months for the mature minded. If your party treats you with respect and encouragement it should be easier for you to accept the realities of your situation within a year. For example, US Navy boot camp is 8 weeks. You receive training on what to do on exposure to the enemy. This is the basic set of time to teach you how to deal with what you're going to experience. If three or four untrained guys are trying to get you to feel better, it may take quite a bit more than 8 weeks to toughen you up for combat.
If they aren't lending a hand to your recovery it should be indefinite (Habitual drinker, saw one too many battles, sound familiar?). Your character may have a crisis of faith, but could continue being a preacher with a different direction to his sermons.
Finally, as a preacher, it's your job to handle death. Unless you are fresh out of the holy box, you've probably handled at least a single funeral. You may also have commended at least one soul to heaven ( or 3.5 equivalent ), and are responsible for protecting your flock from the forces of necromancy/un-death. While you may be green, you most likely were also prepared.
Good luck, have fun, remember to have the rogue check for traps before opening doors.
(Sources: Acting classes, Psychology major.)