It's a moment. Or a thought in my mind. I hear a line of dialogue of mine. It could transpire from anything, as simple as a crushed book of matches on a window sill at the theater. I get them often. Even the most random nutty thoughts.
I worked in theater for years as an actress, so was primed to understand language in character driven dialogue. Various tensions and how a word is like a color that can be the start of a masterpiece or 'tone' of the work. In sketch and especially stand up understanding the behavior of words, how we formulate a character, or a 'scene'. A sentence, like a first great line of a book and it springs and opens it all up. However, the writing isn't difficult. It's then working it, rewriting. I lately, have had many of those moments or a thought start something, and I just happen to shut it off and never write it down. I think I should again, because I am so in love with fiction more than ever now. Which is why I joined this forum. The passion of others who value writing and reading, on any subject, genre or form of writing is so instrumental as well.