James McCormick at 2012-06-21 19:26:04:
A lot of times, the first major Subplot is where you find the heart of the story.
The most clearcut example is BACK TO THE FUTURE. The plot is about a kid who accidentally gets sent back in time. The structure of the plot is his struggle to return to the future. But that's not really what the movie is about -- in fact, the movie has a device that allows them to "pause" this plot for a week to give it time to play with the subplot (the lightning bolt happens at a specific date -- just long enough for Marty to get to know his parents and royally mess things up). Very little of Act 2 has to do with Marty getting back to the future.
While the lightning bolt is the main plot's ticking clock, the subplot has one as well -- The Enchantment Under the Sea dance where his parents are supposed to kiss for the first time.
You see this structure a lot in classic Hollywood cinema -- 1) a main plot driven by action and events outside of the main character that acts as a framing device and 2) a strong subplot that deals with the protagonist's major problem/dilemma, which usually ends up being the bulk of Act 2.
This also fits into Scott's paradigm of the status quo > journey > return to a slightly better status quo structure (which is also very similar to the Hero's Journey). The main plot knocks the protagonist out of the status quo and into being forced to deal with something he didn't want to/or know he needed to confront, which presents the journey.
However, examining it in terms of plot and subplots shows how they intertwine and work off one another to create the structure of a film.
For the movie to be successful, it has to payoff both things 1) Marty has to return to the future and 2) Marty has to set things "right" with his parents (the revelation comes in Act 3 where he discovers he actually improved all of their lives).
Terry Rossio says all movies really need 2 ideas. While I don't personally agree with how he articulated the idea (I think all films are fundamentally about 1 thing), at the heart of what he is describing is this Plot/Subplot relationship.
1) Kid gets sent back in time and must harness the power of a lightning bolt to return to the present.
2) Kid meets his parents as teenagers and must right the wrongs he created by accidentally meddling in their past.
These could be the plots of two separate, entirely different films. The magic of story is finding the symbiotic relationship between these two plotlines.
What happens is that the main plot creates the outer conflict, giving larger, broader stakes, while the subplot delves into the underlying subtext of such a story often dealing with inner personal problems and relationships.
Shane Black articulates this much better imho. I'll hunt around for the quote, it's too buried in Iron Man 3 stuff to find it with ease.
If you look at Lethal Weapon, Up, True Lies, Die Hard (most classic Hollywood stories) they follow this relationship between plot and subplot. (Interestingly, Die Hard is actually backwards compared to the others -- the relationship subplot is the framing device and the main plot occupies the majority of Act 2).
The plot/subplot relationship is vital. How they connect is what makes an interesting hook into a great film.