Top producer/writer Stephen Cannell pointed out that most people plot with their head and write with their heart. I think this is an important clue to what sometimes goes wrong with our writing projects, and how to fix it.
In "head writing" (that is, plotting or planning a writing project) we are in a dissociated state. That is, we are outside the story or the structure, analyzing how it would best work. In this mode, we are aware of the rules or guidelines for the genre in which we're working, and perhaps thinking of the commercial demands of the project as well. These might include how long it should be, the tone of the piece, the nature of the ending, and so forth.
In "heart writing" we go into the associated state, meaning that we enter the thoughts and feelings of the people we are writing about and vividly imagine the actions we are describing.
If we use head writing for both, the outcome is likely to be precise, correct--and bloodless.
If we use heart writing for both, the outcome is likely to be colorful, vivid, passionate--and chaotic.
So, head first, then heart is a good prescription. But sometimes when we're writing, we get the feeling that what we planned isn't working, or doesn't feel right. The problem may be that our heart is alerting us to the fact that what we planned doesn't ring true. Cannell's suggestion is to ask yourself, "If this was really my problem, would I do what I'm saying this character is doing?" If not, use your heart to give your head the answer to what to write instead. Often we find we have overlooked the what the character would really be feeling in the scene. When we get in touch with that, it's easier to figure out what the character would be doing.