Anyone actually says the word 'actually'; double if it's done as part of a Title Drop
Anyone swears
The Billy Mack "Christmas is all Around" song is played
You see a note on Sam's door
There's a story crossover (Daniel talks with Karen, Colin flirts with Mia, etc)
There's nudity (so any scene with John and Judy, basically)
Sarah's phone rings
You spot someone wearing a polo-neck/turtleneck sweater
David says or does something cringeworthy
THAT dance sequence
Jamie tries to speak Portuguese
There is a cameo appearance by a celebrity starring as themselves (Michael Parkinson, etc) note Claudia Schiffer doesn't count, as she's playing a character who looks like Claudia Schiffer
Serious Drinkers' Bonus Edition: All of the above, plus...
Drink whenever someone you're watching the film with does any of the following:
Points out that Natalie is not actually fat
Expresses dislike for Mia (double shot if it includes a swear word)
Points out that Mark's behaviour is actually quite creepy
Compares the President to any real-life holder of that office
Asks if that really is Claudia Schiffer
Refers to any character by the name of a character played by the same actor in another film or TV show (Jamie as 'Mark Darcy', Judy as 'Stacey', John as 'Bilbo Baggins', etc)