1. It was all a dream (Wizard of Oz, Woman in the Window).
2. I’m really dead (Man without a Past, Owl Creek Bridge).
3. You were meant to fail - that’s why you were hired!
4. The fake medium turns out to have real powers.
5. A baddie is the only person the hero can turn to for expert assistance. (And you end up sympathising with and even fancying the baddie. In extreme cases the baddie then gives his life to save the others.)
6. A dork is the only one who has the knowledge that will get the gang out of the mess they’re in.
7. A fop turns out to be a hero (The Scarlet Pimpernel).
8. An eccentric collection of boffins get together in secret location (underground lab, seaside boarding house) to save the world/win the war (School for Secrets, Andromeda Strain).
9. The characters spend the film chasing a suitcase full of money. It ends with notes blowing away in the wind/floating on the water as they lie dead/are taken away by the police/die of thirst in the desert. Previously, there’s a scene where someone picks up untidy handfuls of the stuff and shoves it at someone going “Take it! Take it! I don’t want it!” (Often starts with “These two cops/strangers/married couple/man and woman who’ve only just met find this suitcase full of notes and…”.) No Country for Old Men, Too Late for Tears, The Pardoner’s Tale.
10. A meek, colourless girl moves in with (on) warm, outgoing, popular, pretty girl and gradually takes over her style, friends, catchphrases, boyfriends, flat, job. Popular girl is left an overweight, alcoholic, neurotic reject. Vanity Fair, Single White Female, Three Women, All About Eve, Bernice Bobs Her Hair.
11. Two women are life-long sisters/friends/rivals. When one’s up, the other’s down. One writes serious novels, the other decides to write historical trash and becomes more famous/rich. One steals the other’s husband. The other steals the first one’s daughter. Half-way through, the more dowdy, serious one is leaving the room when she turns round, comes back, puts down her bag and gloves, and slaps the other across the face. (Old Acquaintance with Bette Davies and Miriam Hopkins.)
12. The hero/heroine finds happiness/moral integrity by returning to third-rate vaudeville circuit after finding fame on Broadway because he/she does it for sake of mother/father/husband/wife.
13. A young ingenue joins the act and an old vaudevillean teaches her all he knows (Sell it! Eyes and teeth!). She auditions for a Broadway show and gets the part by ignoring the old vaudevillean’s advice. Cut to him looking miserable/he returns to circuit and shoots self while listening to her hit record/their old number.14. The detective did the crime but doesn’t know because he’s got multiple personality disorder, amnesia etc. (He solves the murder by following the clues he's left for himself.)
15. A doctor works all hours in a laboratory in his back garden to create a new wonder serum. He may go mad and start shrinking people.
16. The killer leaves crossword type clues for the investigating team (“He’s copying famous crimes! He wants us to guess where he’ll strike next!” “The clues are all in pairs!” “All his crimes are based on Dante/the Bible/Shakespeare!”) (Theatre of Blood)17. After staggering clear of the car/train accident and being presumed dead, the former rich industrialist ends up in Smallville where he strolls into a job as a mechanic. After eating a lot of home-made fruit pie, he finds happiness with the girl next door. Given the chance of reclaiming his old identity/millions, he smiles wryly and stays put in Smallville.
18. The man falls for the girl when she's come to New York because her sister’s disappeared/been murdered and he gets roped into the search. Ie attracting someone is the last thing on her mind and she's not trying at all but being emotional, rumpled, honest, wry, bitter etc. and doesn't have time to be brittle, coy, cute or flirtatious.
19. A basically decent, idealistic, young (man/woman) goes to (New York/Chicago/Los Angeles/D.C.) to make his/her mark in (writing/business/music/ acting/government) only to be temporarily seduced by the very environment/person they are the antithesis of, alienating his/her (boyfriend/girlfriend/ family/friends/all of the above) in the process until he/she stumbles on to the revelation, "To thine own self be true." IMDB on The Devil Wears Prada
20. A disparate group of characters yoked together by a quirk of fate. (Peter Bradshaw in the Guardian)21. A cute, sad maths genius solves Fermat’s last theorem to show his/her dad how much he/she loves him, or to show us that very clever people are really just dim people underneath, or to show that it’s all right for people to be very clever as long as they suffer. If a maths genius, you must have a disability or a trailer trash mother. Movie must have the word mind in the title. (Agora with Rachel Weisz as medieval mathematician Hypatia has just premiered at Cannes.)
22. The US president is going to be assassinated. May or may not involve psychic foreknowledge. (Manchurian Candidate)