Imagine your story is a game of Black Jack. Or a Game of Texas Hold'em. The firsts cards are important, it is good to have a nice first draw. But, the most important thing is that, if you decide to play, you have to play with cards you have, and next cards you draw have to work with what you already got.
Lets call each card an "element" of the story. A collection of elements give the story it's tone, it's subject matter, it's genre, it's uniqueness. The elements are the stories DNA. A man is an abstract element. An old man, more specific. The fact that the man is old already limits the possibilities your story may go, it specifies a path. A marriage is an element. A troubled marriage is an specific one.
It's hard to define what constitutes an element. It has to do with specificity and singularity. The more specific and singular, the more elemental.
The more specific and combined the first elements are, the more defined and limited the possible paths your story may go. Lets say element 'Man' has X possible paths. Element 'Married Man' has X-'stories that make sense only for bachelors' possible paths. And "Old Man" on "Troubled Marriage" has X- ['stories that make sense only for bachelors' + 'stories that makes sense only for young people', + 'stories that make sense only for happy functional marriages']
There are no rules for choosing the first elements, the rules come when you decide to play with them. But the process is messy. There is a chicken and egg situation between theme and elements. They feedback each other but the more you play with them, the more clear everything gets. Once again, poker: I have two aces but I also have the possibility of a flush. Hmm? Should I let the Ace go and play for the flush?
You will find out that some elements work and some don't. The criteria for choosing what to keep and what to take out depends on what you want to tell [what is the story about]
Elements and sense of story [what you want to say] are the basic blocks, next blocks are usually time and knowledge. Time is time. What happens before and after. Knowledge is the relation between time of the fiction and time of the storytelling. It has a threefold constitution: what the reader/viewer knows compared to what the characters know, and what the narrator knows. What the author knows most be treated with caution and used as little or smartly as possible. [Holden Caulfield as narrator knows everything that is going to happen but he does not know what Sallinger knows about him. He is not suppose to know it.]
Lets call this knowledge constitution notional space. Think about Texas Hold'em, there are cards everybody has seen, cards only one player and the audience has seen, and cards nobody has seen. The result is what usually is called: characters, action, structure and plot. But this results are results. It is the outcome the audience appreciates, they are not, they are not the building blocks of the story.
Several elements travel towards a sense in time. Storytelling is basically the art of making this travel interesting and meaningful using notional space as the principal tool. Notional space will keep on fixing elements. Will fix language, voices, tone. By rule characteristics defined are expected to be preserved and if changed, the change is expected to be justified. Notional space also by rule defines the main character we are traveling with, as it usually the character who knows exactly what we know at any point in time.