mack,

the prematch screening is partially to determine your readiness to leave behind addictions to unnatural substances and to sensual recreations. it is a week and a half of hard, very hard work, and they want to see that you are not doing it as a cure or subsidy for sensual addictions. also, it protects them (the facilitators): they are simply volunteers; one assistant teacher and a number of "old students" who serve the course. no one is prepared to deal with those who have serious psychological problems. as vipassana can bring out deep-seated issues, they want to make sure you're not a psychotic episode waiting to explode, so to speak... i wouldn't worry much about the drugs. we all have sensual addictions - it's the fundamental recognition of vipassana, and of buddhism. drugs are just an illicit form - a proper student of vipassana seeks to rid one's addiction to all cravings and aversions, which are only caused by sensations (natural, circumstantial, or chemical).

as far as posture, i'm tall and skinny, so i find most postures uncomfortable, as i have very little meat and padding to support my lanky bones... the seza sitting position is comfortable to those who regularly sit that way, or are flexible. it's sitting on top of your feet, with your knees in front of you. i find this position most uncomfortable. the bench is essentially the same sitting position, but takes your body's weight off your feet and lower legs, so it's more comfortable. i find it about as comfortable as sitting cross-legged, which is how i ended up sitting the entirety of the course i just finished yesterday. my first course, i tried all the positions and "pain management paraphernalia," this time i made do with a couple cushions and a little folding seat (just a flat base with a stiff lumbar back) and sat cross-legged. it still hurt a lot, and that's the point of the exercise - purposely expose yourself to pleasant and unpleasant sensations, learning to become equanimous with them (meaning, train your mind not to react with craving or aversion). it's really hard work; it will not be comfortable.

your first retreat will be a wonderful experience, nothing like what you likely expect. note that it's especially hard day 2 and day 6, so keep the strong determination to not give up, especially those days.

may your find your peace and happiness