When people ask me how I know whether an idea is a play, a film, a musical, or a TV show, the answer is pretty simple. Mostly, it's an intuitive feeling; the ideas tell me how they'd like to be written. My belief system about mediums:
Plays - stories in rooms the characters can't leave
Films - stories about one character against her landscape
TV Shows - stories about a world
Musicals - stories about two worlds coming together
I love all mediums of dramatic writing and use each one to accomplish its unique strength, like using a hammer for a nail instead of an ice pick.
My stories in rooms that the characters can't leave (hint: neither can we).
My stories about characters pitted against their landscapes.
My stories about communities of people for us to wander around for years.