Basic Trope: Someone only acknowledges their feelings for somebody else after seeing that person get romantically involved with someone else.
Straight: Farah claims to be Just Friends with Mark; however, when he hooks up with Lucy, she gets extremely jealous and realizes how much she really likes him.
Exaggerated: Farah doesn't even seem to be aware of Mark's existence, even though he clearly has a bit of a crush on her... But when he starts dating Lucy, suddenly she's hopelessly in love with the guy.
Justified: Lucy is Farah's friend, and Farah never noticed Mark until he started dating her friend.
Inverted: Mark only realizes his feelings for Farah upon hooking up with the shallow, bitchy Lucy.
Farah denies having any kind of romantic feelings for Mark. Then she sees him with Lucy, learns that they date, and it's obvious that she's jealous... but in the end of the episode/chapter turns out it's because she fell in Love at First Sight with Lucy.
Double Subverted: Only to reveal she only said this at Lucy's request to see if Mark was serious in their relationship, and Farah really does have feelings for Mark.
Farah becomes intensely jealous of Lucy, because she wants all the attention Nameless Boy A (or Matt, or whatever) gave her.
Farah falls madly in love with Mark when he starts dating Lucy. When Mark notices this and breaks up with Lucy, Farah becomes completely uninterested in him.
Once Farah finds her feelings for Mark, she attempts Operation Jealousy on him with Jeff. Only for him to attempt it again with her with a Carol. Farah, considering her operation unsuccessful, tries it again with Matt. And so on.
Farah develops feelings for both Lucy and Mark, and joins them in polyamory.
Farah really doesn't have feelings for Mark and is happy that he met someone.
Exploited: Mark only hooked up with Lucy to get Farah to come around.
Defied: Mark, after briefly considering Lucy, declares Farah his one true love.
Discussed: "I didn't notice him until he went out with Lucy— that bitch!"
Conversed: "Then he goes out with this other chick, and the first girl becomes jealous!"
Deconstructed: Farah only wants Mark now because he's unattainable; if the status quo is restored she goes right back to claiming they're Just Friends, ignoring or belittling him. Mark figures this out and decides to stick with Lucy, who he hopes won't jerk him around and abuse his feelings.
Reconstructed: Upon Mark's return to Lucy, Farah discovers her feelings for him. Lucy and Mark break up amicably, and Mark winds up in a relationship with Farah.
Played For Laughs: Farah is over the top in her denials of any attraction to Mark until she learns that he's dating Lucy, at which point she instantly morphs into a Clingy Jealous Girl.
Played For Drama: Farah has denied her feelings towards her best friend Mark for years, but now... he's gotten engaged. To Lucy, who seems perfect and wonderful for him. Yet Farah can't deny her jealousy, and struggles with how to react.