Great scenes are the building blocks of your story. If you're writing horror, they should terrify your reader. Sci-Fi and Fantasy should pull the reader into the world you've created. Romance should make their hearts swell.
Generating an emotional response is the key here. You want your reader IN the story, experiencing the ups and downs of characters they love, and in some cases, love to hate. Every scene has to count when you're writing a story that you hope to sell and every word counts.
Thinking in terms of comics, you have a limited number of pages per book, panels per page and, assuming you don't want your artists' hard (and expensive) work covered up with word balloons, a limited number of words per panel. Working within those parameters and becoming accustomed to them can really help you focus on which scenes, panels, and lines are critical to the story and which can be cut.
Everyone has their own style, but I like to break down a scene into four major components when I'm writing.
LOSS/GAIN - The hero in your story is surely after something. Some goal that drives him/her into action. Capturing the attention of a love interest, getting access to a safe full of money, finding a way to get back over the rainbow, etc. Increasing the stakes for characters that your readers care about, pulls them in closer and holds them tighter.
Let's take a look at my book, BLACK OF HEART, as an example. In issue 1, our hero, DRAKE, is struggling to find a killer in 1940's New York City. There was no CSI or forensics in those days and string of random killings was hard to trace. He's put in another all-nighter and realizes that it's Valentine's Day and he should probably get home to his wife. When he shows up early with a bouquet of roses, he's surprised to find her in bed with another man. OUCH. That's loss on big scale - a kick right to the heart. And while, yes, this is story about a killer with a black heart, it's also about a series of events that leads Drake on a path that blackens his own heart. He fights his wife's lover, getting punched in the tooth, causing some major pain, (this moves the story forward later in the same issue) - again, LOSS. Not only is his wife cheating on him, but he just had his manhood handed to him by the man she was cheating with. Now that she's busted, Drake's wife, PATTY, has her own goal - to make him feel small, hurt and to reflect some of the guilt that she feels back at him for leaving her alone all of those days and nights while he was obsessing over the killer that he can't find. Now a shell of a man, Drake throws the Valentine's Day roses at his wife as he walks out, unsure of where he's going - only knowing that he can't stay there after what just transpired. Another loss for our hero and a loss for Patty, who is cut by the thorns of the roses he threw. Not only is Drake failing at his job, he's failing at his marriage too. And so the downward spiral begins into the deeper drama of our story. It reveals quite a bit about the character of both Drake and Patty and begs the question - what will happen next? Something you definitely want your readers asking.
DIALOGUE - Above and beyond a memorable one-liner, the dialogue of your characters can tell the reader WHO they are, WHEN they are, WHERE they are, WHY they're there and WHAT they're after. Maybe it's an internal monologue, or a shouting match or comic relief, breaking the tension of a scene. Keep your dialogue true to the character that's speaking it. I see this a lot, where writers have similar voices for multiple characters. You end up with a boring story that loses the reader's attention. Avoid exposition and pummeling your reader with details to fill in plot holes or as an attempt to add depth to your character. Actions speak louder than words and you'll have to get creative to find ways of making your characters larger than life. As I mentioned above, if you're writing comics, your words have to count because space can be limited.
Sticking with our example in BLACK OF HEART, let's look at what the dialogue reveals about Drake and Patty. Patty's lover has exited stage right after punching Drake in his bad tooth and he confronts her directly.
DRAKE: YOU HAVE ANYTHING TO SAY FOR YOURSELF?
PATTY: THANK HEAVENS YOUR HERE, OFFICER. MY HUSBAND'S BEEN MISSING FOR WEEKS!
Since Drake is, in fact, a detective, the "officer" quip is funny and ironic, while her follow up cuts deep, essentially stating "you're never here, jerk". In other words, she went for the throat, and as I mentioned previously, tried to deflect some of her own guilt upon him, practically blaming him for driving her to cheat.
Drake grabs the roses he brought her, tosses them at her and leaves.
DRAKE: HAPPY VALENTINE'S DAY, BABY.
It might as well have been a slap in face, but that's not the kind of man Drake is. His actions tell us that. And while these lines don't exactly tell us WHEN this scene occurs, (outside of V-Day - I'm speaking mostly of the time period), there aren't any modern words used that would absolutely define a specific decade. For example, if Patty had said "Chill out, dude", you might guess it was in the 1990's. It certainly tells us WHO they are - again, Patty tries to hurt Drake and he, in turn, walks away from the situation. What they WANT (and hopefully the reader as well) is resolution. She tries to instigate a fight (or conversation) about what happened and he wants to escape because he can't deal with the reality of it. Literally writing that dialogue out - "Hey, I want to talk about this..." and "No way, baby, I can't handle this right now..." is less impactful in my opinion. You don't need a long string of dialogue to get the point across and there are gaps for the reader to discover and fill in on their own, making them more involved in the story. Keep it interesting!
ACTION/CONFLICT - I don't mean action in the sense of a big car chase followed by a massive, Michael Bay-style explosion. I'm talking about the actions of our characters in a given scene. Building conflict. Driving the story forward. Yes, there's a short-lived fight in the scene we're examining, but delving deeper, beyond that are the actions of a husband and wife. One guilty of never being home, the other guilty of needing to be loved - in this case she gets that attention from another man. Without the fight scene, you have a man and a woman having a conversation about their lives together. Their words give some insight into their actions. Even though we're only with them a short time, we have a higher understanding of what their relationship is like - and it's not good. We can see what kind of people they are on a deeper level. It's this type of revelation that your readers will identify with, that pulls them into the story. Maybe you can identify with Drake, who just had the rug pulled out from under him, or maybe you can identify with Patty, who just got caught doing something morally wrong, even though she's found her own way of justifying it. These actions help to define the characters in this scene, increasing the level of conflict and pushing toward a resolution. Either they work it out and get back together, or they go off in opposite directions on their own separate paths. Hopefully readers want to know how it turns out. If you've written a powerful scene, they will.
METAPHORS/SYMBOLISM - Someone has just died and a candle blows out, reminding us how precious and delicate life can be. The strong current of an ocean or river hints at trouble beneath the surface of your story. There are thousands of examples. During the outline phase of any story I write, I look at the themes and do a bit of research into the symbols for those themes. Then I try and find ways to use them naturally in the story. You'd be surprised how often these types of things happen on their own without purposefully injecting them. It's safe to say that most writers have absorbed a great deal of story - be it movies, books, plays... whatever. Parts of those experiences happen on a subconscious level that ignites something, bringing a scene (or an entire story) full circle.
In our BLACK OF HEART scene, let's look specifically at those red roses. It's Valentine's Day and roses represent love and passion. Kind of a no-brainer. But what about devotion? Ironic in this scene with an adulterous wife. What about the thorns along the stem? They represent the rose's only defense from an animal (in the wild) that may want to eat it, or a human that may want to pluck it, in essence, ending it's life. In this scene, Drake leaves, tossing the roses at Patty and sits in a sobbing heap on the ground. A metaphor for their relationship. He's literally throwing them away as he walks out. The thorns cut Patty and her blood is the same red color as the rose. She has a physical wound to symbolize her internal wound and the color itself is symbolic of any number of things including sexuality, strength, or anger. There are layers upon layers here to be examined and one reader may pick up on the color, while another finds the irony. The use of symbolism in your scene can enhance the experience of your readers. Pick up a symbolism dictionary and do some research against the themes of your story. Experiment and find something that works great in the scene that you're writing. Try a few and pick one or two that work the best.