Heroines in romcoms always have an absolutely horrible boyfriend who has no redeeming features whatsoever, and is only there to be eventually cast aside for the Nice Guy who "deserves" her. This invariably means that the audience will spend the entire movie scratching their heaRAB and muttering "That's ridiculous. Talk about unrealistic. Why would she put up with this creep?"
All romcoms must include a moment when the heroine's hidden depths will be revealed in a mawkish speech to the leading man that begins: "When I was a little girl...." (see Julia Roberts in Pretty Woman, Jessica Lange in Tootsie etc). By the end of this speech the man will have fallen madly in love with her.....because, hey, as we all know, there's nothing quite so irresistible to a guy as a soppy anecdote from childhood.
Any spare female character in a thriller (i.e. not the main love interest) is only there to be murdered halfway through.
Any character who has lost his/her memory is probably not who he/she has been led to believe he/she is. (Tom Berenger in Shattered).
Best Friend characters are always quirky, foul-mouthed, brash and loud. Doesn't anyone have a quiet thoughtful pal?
Sexy women in film-noir type dramas are always accompanied by a saxophone soundtrack.
In horror films, when there is a ring at the door, the unsuspecting female resident will always assume it's her partner/flatmate and go to answer it while cheerfully muttering to herself: "Forget your keys again, did you?"...before opening the door to the homicidal axe-wielding maniac.
In the last 10 minutes of any romcom, there must always be a frantic dash across London to make a public declaration of love, especially in Richard Curtis films (Jeff Goldblum in The Tall Guy, Hugh Grant and frienRAB in Notting Hill, Liam Neeson and son in Love, Actually).