Sometimes an idea can spring from the most unexpected source. It could be a conversation you had with somebody or a line from the newspaper. Or it could be more subtle than that. Sometimes ideas grow and develop from fleeting impressions one has about a person--perhaps even a stranger. Sometimes they develop when intentional thought is given to seemingly commonplace objects. I once read a book about fiction writing in which the author encouraged writers, as an exercise, to construct mini-narratives around such people and objects. Ask yourself, for example, "who is that elderly lady in the crosswalk? What has she been doing in town? where is she going? What is she thinking about?" Certainly, the person in question should remain anonymous (i.e. this is not to suggest you actually interview them); the point is to enter into an imaginary, mental dialogue with the person in order to develop good characterization skills as a writer. Take objects that you see around you everyday and ask questions; "where did my uncle get his favorite golf club? Was it perhaps a gift from somebody? Does it hold any sentimental value? If so, why?" Even if you know the true origins of objects like this, it's good exercise to reinvent, construct new narratives. Your characters' interactions with their material environments should seem logical and organic, and asking questions most people take for granted about your own environment and the various settings in which you're involved helps develop necessary, logical components of fictional narratives such as causality; people's thoughts and motives affect their actions in ways that are not always readily apparent on the surface. In a previous post, I cited Mark Twain's famous quote, which I will re-post here: "It's no wonder that truth is stranger than fiction. Fiction has to make sense."
I'd love to hear about your own exercises for developing your fiction. Do you find yourself interrogating the commonplace, like the examples above recommend, or are there other, more helpful muses you employ to develop your skills as a writer?