Troublesome abundant ideas

Every time I sit down to try and write something short and sweet it gets bigger. Just taking a moment to flesh out character can mean that, within moments, I have his or her entire life planned out and don't want to leave out a single incident. Before long, the short splendid idea I had suddenly becomes longer and rapidly spins out of control or worse I realise its full of unrelated filler.
However, sometimes it has unforseen and beneficial developments.
I have five main characters of my new project all with back story and circumstance who are all about to come together and emerge changed. For no reason that I can fathom, this morning, while riding the train I had an image of two of them attending a performance of The Browning Version. My first reaction was to dismiss this as ridiculous but them more I thought about it, it not only would fit but would be a nice scene to develop the two characters relationship.　I once saw my father