Three of my favorite trailers are for: The Fountain, Across the Universe and The Butterfly Effect. I've still yet to see Across the Universe, I had difficulty wrapping my head around The Fountain, and The Butterfly Effect isn't really anything to write home about.
Despite this, I regularly rewatch the trailers for all three films as a package that exemplify how love stories often "work" better in trailers than in film. Okay, I don't mean that literally; what I'm saying is that it's easier to be affected by a love story in a trailer than in a film.
Or this could just be me. I'm almost embarrassed to admit that when I watch each of these trailers I feel sad, and find myself thinking:
"Oh no, poor [character names]; It's terrible to love someone, but be forced apart by circumstances, and your own decisions! 😭" I feel especially silly when I find myself thinking: "Oh no, poor Ashton Kutcher!" but it still happens.
Why do these love stories work in trailers, but frequently are less convincing in their films?
It's because while trailers are constructed via longer films, they call for a different style of storytelling. Trailers are more effective when the scenes are very intense. That's why all the montage shots are people screaming towards the camera, tears running down cheeks, people getting punched, car crashes, explosions, hugs, slaps, and closeups of disrobing people. Trailers have permission for everything to be incredibly dramatic because that's what the audience expects.
Of course, trailers need to vary their pacing to be truly great, but since they're so short, the audience will be more forgiving if it's nothing but bombast for two and a half minutes. When a feature film sustains that level of energy you get something like Michael Bay's Transformer films which are positively exhausting. (For more about Michael Bay, I HIGHLY recommend Lindsay Ellis' video essay about his style of directing)
Scenes with a lot of subtlety might require the full context of the film, or enough time to play out for the audience, but trailers don't have that luxury. It's much harder to glean the meaning of a subtle look on an actors face when it's on screen for such a brief period of time. When the audience expectation is a high level of drama, anything less than that either doesn't get processed by the brain at all, or just seems boring by comparison.
Just because a scene of yelling or crying works in a trailer, doesn't mean that it'll work in the context of the film. In a film, scenes that like that can easily feel unearned, cheesy, eye-rolling, or over-acted. Trailers are a case where lack of context can be a benefit, because the audience will do their best to connect the pieces, and imagine the rest of the story.
We've probably all seen love stories in films with cliched elements that turned out great because of a competent director, writer, and actors. So we know intellectually that just because there are cliche bits in the trailer, doesn't mean that the entire story will be the same.
All three of these trailers are about people in love who are forced apart for different reasons. The emotions shown are big, and raw.
Of the three, The Butterfly Effect seems to show the most plot. It didn't have anything else to lean on because the story and presentation is the most cliche, so it kept showing more, and left you with the question of whether or not there will be a happy ending.
Across the Universe falls somewhere in between with its boy meets girl story where when the Vietnam War happens, and her interest in activism conflicts with his desire to stay at home, and focus on his art. Also, everything is surrounded by Beatles songs, which depending on your taste for the covers will either attract or repulse you. The question you have at the end of this trailer is the same as The Butterfly Effect, but on top of that the question is "What's up with all this music video-like imagery???"
The Fountain pretty much has the audience asking questions from the first few moments; we have no idea how these three timelines connect. Are Rachel Weisz and Hugh Jackman time travelers? Are they being reincarnated? Regardless, the idea of a love that transcends time and space sounds intriguing, and we still want to know how it all turns out. Will they find the secret to immortality, and will Rachel Weisz survive whatever ailment she's suffering from? Also, what the heck is going on???
These three love stories show us the major highs and lows of the relationship, with varying amounts of connective tissue or context. I think these work as mini stories because of our tendency to fill in the blanks with our prior life experience, and film knowledge. Who knows, maybe what we imagine will be better than what was actually left out of the film.
If you're editing a trailer, keep an eye out for this kind of dialogue, and visuals when making selects. The less context required to understand a shot or line of dialogue, the better. The more lines of dialogue you use to explain a story point means a longer, and less effective trailer. Guide the audience through the basics of the story, but let them connect some dots, or wonder how they're going to be connected, and they're more likely to show interest!