Becaue you havnt given specifics, neither can I. No that I want to write your script for you.
-foreshadow the twist so you dont cheat the audience. give several subtle clues or hints without actully making it predictable - not easy at all(million ways to do this, but it is a balancing act)
-make sure everything is integrated. otherwise may seem contrived.
-scripts success shouldnt rely solely on tiwst. otherwise its just a gimmick. script should be successful even without twist.
-twist better be logical and not as the result of amazing coincidence.
-twist should be original, lots of "twists" out there (seems to be very popular these days, everyone wants their script to have twist, plenty of great stories do not, not that twists are bad - Im just sayin)
-rarely do people react to a twist the way you want, like "oh my god thats BRILLIANT" it happens rarely. sure theres some recent movies, but theyre the exception - keep that in mind. If people detect that your trying to manipulate them, theyll turn against you.
Once you write it and ask people to read it, dont mention at all that there is a twist. otherwise theyll look for it.