Before I write a scene I have to spend some time thinking about how I want to approach it. I may know what's going to happen next in my story. I may have my little index card in front of me that tells me who is in the scene and what the scene needs to accomplish (the "plot point") but that doesn't mean I have the scene figured out yet.
I call it cracking the scene: figuring out the hook that allows me to execute the plot point in a way that is interesting to read, true to the characters and logically flows from what has come before. I have cracked scenes in several different ways but usually the first thing I do is find a way to make it as exciting as possible.
For example, when my index card told me "Lucas joins the army", I ended up with a scene depicting Lucas surrounded by bad influences, falling to temptations, resisting arrest, getting beaten by enforcers, thrown into shackles and pressed into military service against his will. In other words, I believe I took the simple note about the plot point and punched it up considerably. And at the same time, commented on the character of Lucas by showing what sort of people and situations he allows himself to be around.
But punching up the action is just one aspect to cracking a scene. The primary choice I believe a writer has to make in presenting a scene is choosing the POV. I'm writing in limited third person, so who is going to be the point-of-view character for a particular scene? My answer is informed by two factors: 1) Whose POV will be the most interesting? and 2) Is there a character whose POV is revisited regularly through the course of the book?
In addition to POV, the choice must be made when to pick up the action of a scene. Should it start in a quiet moment? Should it start in the middle of action? Should it start in the middle of conflict? Should it start with a line of dialogue or a descriptive passage? There will be no consistent answer to these questions, nor should there be. The variety of presentation will make the story better.
Sometimes, if I'm stuck on how to open a scene, I have a trick that usually helps me out. That is, focus on one of the five senses. What does a scene sound like? Or smell like? What's the POV character's reaction to it? That's usually enough to get me started. Hopefully, it succeeds in pulling readers into the scene, helping them to visualize.
Looking back on what I've written, I can point to another emerging pattern in my style. That would be one of symmetry in structure. Whatever I open with, I try to revisit it at the end of the scene. I find this usually provides a pretty solid spine for the scene. For example, I opened one scene with a character studying the night sky, he turns away and other things happen but I end with him commenting on a constellation. I think I've heard this concept called "callbacks". Stuff like that seems to put a nice little bow on the end of a scene, providing a complete miniature reading experience.
Cracking a scene. I start with a plot point note on an index card, then make a series of decisions (regarding action, POV, starting point, sensory hook, closing callback, etc.) and before I know it I have what I consider to be a pretty strong scene.