Basic Trope: In romantic comedies, the character who gets dumped by the Love Interest.
Straight: Alice and Bob are engaged, and there doesn't seem to be any problem at all. However, when Charlie enters the picture, Bob slowly turns into an incredibly jealous, inattentive Jerkass so Alice can dump him for Charlie.
Exaggerated: The very moment Charlie enters the picture, Bob goes from an incredibly all-around nice person to a raging jackass at the drop of a hat.
Justified: Bob always had these flaws, but Alice didn't realize it until she had someone to compare him to.
Inverted: Alice hates Bob and is only with him because of an Arranged Marriage. However, once Charlie shows up and romances her, Bob notices how much of a jerk he is and changes for the better so dramatically Alice falls in love with him for real.
Charlie's appearance brings no change in Bob...
Alinor's Nice Guy fiance Bernardino is hanging about, losing her to the dashing Constantine, when she discovers that she doesn't know him because she has never tried to, and he's full of interesting Hidden Depths.
...for a while.
. . . but not romantically inspiring ones.
Parodied: Bob is an incredibly friendly and devoted fiance when Charlie isn't on screen, and becomes a jerk the moment he appears.
Zig Zagged: Bob turns into a douche when Charlie appears, but that's because he hates Charlie, except that's all Vitriolic Best Buds, but it turns out Bob really is a jerk underneath.
Averted: Bob doesn't change his attitude when Charlie shows up.
Enforced: "Alright, so we've got our romantic comedy all set up - we've got the handsome lead, his love interest...oh, crap! She's engaged, right? Let's just turn the fiance into a jerk!"
Lampshaded: "You haven't always been this rude!"
Invoked: Charlie tries to get Alice to see the bad side of Bob by bringing it out in public.
Defied: Bob is jealous of Charlie, but he decides that's just natural and gets over it.
Discussed: "Bob, are you trying to drive me into Charlie's arms on purpose by acting like a jerk?" "That's what my parents did, and their loveless marriage is still rock solid at rock bottom!"
Conversed: "Y'know, I think the scriptwriters in Hollywood all want to leave their fiances and are working it out through their characters, just going by sheer numbers."
Falling for men on an emotional whim eventually costs Alice, as she gets jerked around time and again.
Alternatively, being dumped from a long-lasting relationship takes its emotional toll on Bob, and Charlie's overt attractiveness proves fleeting.
Charlie really is the right man for Alice, and Bob eventually comes to realize it as well.
After Alice leaves him for Charlie, Bob realizes how shallow their relationship was, and how little effort he put into it. He resolves to become a better and more realized person; in time, he finds someone to love for real.
Plotted A Good Waste: The story takes Bob's point of view, and shows how his insecurity over Charlie makes him fall back to bad habits he'd thought resolved. Rather than let Alice know what he's going through, he pushes her away by attacking Charlie and accidentally hitting her, ultimately loses the love of his life. As Alice and Charlie drive off happily into the sunset, he collapses crying over the Tragedy he brought on himself.
Played For Laughs: Once Alice and Charlie get engaged, Dylan enters the scene.
Played For Drama: The story starts with the breakup of Alice and Bob, and follows Bob through an emotional breakdown and shows Alice and Charlie in a loveless marriage. Because hey, not every impassioned spur of the moment decision turns out to be the right one. Eventually, they end up dead, crazy or alone.