Each dialogue is an action. These are not two different things, but one. Consider the following: action refers to doing something. Dialogue refers to talking, and talking is actually doing something.Moreover, a dialogue can start when characters do something else, like driving a car, making breakfast, etc. The thing is that characters don’t just stand still and talk. It's not like they are puppets in a theater. The task of an author is to find language that works well with movement and motion.
Description works best when it's subtle. When there's too much of description it gets static. Describing is more like painting, not working with a hammer. All probably know that painting's often done in short strokes, very delicately. It's enough to pick one to two-three details and stop right there.
It is completely up to you what is there in your book for the readers, whether it's a psychological thriller or a romance. Still, your readers want to work. They wish to fill in the unmentioned details, make their own investment in it, make their own decisions and conclusions. Don’t write everything. Leave some pieces out. These could be parts of a plot, some dialogues or descriptions. The reader's imagination matters just as much as writer's. Make them feel important and appreciated.
Sometimes, novels have way too many characters. Not that there is a set number, still it's limited by author's ability to make them realize their full potential, as if they were real people. If there's not enough room or time to give them a soul, then it will be better off without them.
If a story is good, it's good. The same with novels: a good novel is a good novel. You just write the novel you want to write without thinking much of its genre. Eventually you find yourself in a situation where you have to think about it. The thing is that bookstores, agents, publishers — they all care about is genre. A writer must make his/her book fit somewhere, although, this fit doesn’t have to be perfect.
The beginning of a novel is relatively easy, so is the ending. This is the middle where it all gets complicated. You can manage it in a few ways. First of all, any new beginning and early ending is a peaks and valley of narrative. Next, there got to be some pressure on the story, in order to make it interesting to read. Finally, treat the second act like there's two more in it.