Though I am never exactly "blocked" I do have difficult periods. I am led by a fascination with material - the challenge of presenting it in an original and engaging way. I have no problem imagining stories, characters, distinctive settings & themes - but the difficulty is choosing a voice & a language in which to present it.
Characters begin as voices, then gain presence by being viewed in others' eyes. Characters define one another in dramatic contexts. It is often very exciting, when characters meet - out of their encounters, unanticipated stories can spring.
Before I undertake a lengthy project, I have usually given much thought to it over a period of years. My files are filled with likely subjects - which perhaps, one day, I will develop.