There are various arguments on how you're supposed to craft fiction. Oh, you start with a plot idea, don't you, and take the character from there? Or you get a character and tangle up the plot from there? The apparent rules to writing fiction are both baffling and irritating. But what about those characters, usually secondary ones, who you can lift from one situation and put into another? They're not linked to plot and they just plopped into your head fully grown. You have to use them but you don't quite know where they belong yet. But, it's alright, because due to their versatility you can play around with them until you're satisfied.
I've got two characters like this. One, Mervyn, emerged from a rather amusing train guard I encountered during my trips from Lincoln to Wakefield during my undergraduate degree. He sang, he helped you out: he was the most good-natured train guard I've ever come into contact with, and I knew I had to take his premise and put him into a story. However, I tried him in one short story - about trains, funnily enough - but the story didn't work. Mervyn did, however, along with the little cast of characters I assembled around him. So I gathered all of them up and changed the transport system - they were now working on buses. Hmm, that story stalled for a good two years. Then, quite recently, I had an epiphany. I don't think Mervyn was happy in short stories because he's a much richer character than that. I know him; I know his speech patterns, his background and his temperament. I know his lifelong infatuation with schoolmate and now colleague, Linda. I know that he's a father figure to a younger workmate, although neither of them would admit it. In short, I know how to fit him into a novel. And best of all? I've got the plot as well. I haven't got the time to write the thing but we'll deal with that problem later.
My other character is one who made an appearance in the first draft of one of my (almost) completed manuscripts. Giorgio was universally disliked by the people who read that draft and, after thinking about it, I chopped him. However, unlike my test readers, I liked him. He was a nice old gentleman, obsessed with art and wanted dead by his money-grabbing son, Patrick. I haven't yet found a place for him, but I will.
To return to my Seven Brides for Seven Brothers analogy, I think that characters who are like Millie's stew are pretty rare. How many characters could you take from one story and substitute into another? We weave plots around character, build character from plot. In my experience, those characters who can stand on their own feet are rare and should be treasured as such.