Writing a three act story - dealing with the middle act. In the second or middle act, you get to expand the story past the initial 'inciting incident', and develop the characters and their various drives and needs. Scenes are a good way of writing the middle act. In the same way as a 3 act story itself, a scene is a 'fractal' mini-version - it has a 'purpose' (the scene's inciting incident, i.e. what the point of the scene is), an obstacle (a conflict to make it interesting) and a resolution. Action and dialogue are the way to go here. Never write flat prose simply explaining what's going on, describe it in actions and dialogue. For example, "Harry Potter was frightened. He'd didn't like spiders. This spider was gigantic" is frankly rubbish. "8 pairs of shining eyes appeared in the gloom, in a rough semi-circular shape. Against the dim light of the moon, Harry also thought to see legs - lots of them. Maybe eight. 'Harry - we need to get out of here - now!' said Ron, tugging at Harry's sleeve. Harry's mouth was too dry to answer, and his legs were turning to jelly, literally." is much better.
Don't constantly surprise your audience by omission. In other words, if Harry Potter can only escape from the spider's lair using a semi-intelligent Ford Fiesta, it's no good dumping it in the scene 2 minutes before you need it. Introduce it well ahead, so your audience won't start thinking 'that's cheating'. Also, make your characters realistic, no matter how outlandish the scenario. Your audience need to be able to identify with them if they are to feel any empathy. It's been said that "Drama is people doing amazing things for sound reasons. Melodrama is people doing amazing things for unconvincing reasons". Suspension of disbelief can turn to outright incredulity unless your characters act from motives that are familiar and believable to the audience. Characters can be difficult to come up with. The character generator at www.GetPlotted.com allows you to click a button and instantly generate a fresh character from literally billions of possibilities. Name, age, physical appearance, motivations, family, all this and more can be created automatically. You can then 'tweak' any part of the character you like in order to make him or her perfect for the part they play in your story.
A good plot, like a good song, grows at it progresses. Challenges should get increasingly difficult until the final resolution. The solution to a previous problem may set the scene for the next problem. For instance, Bill may jump into the water under the pier to escape the attentions of the octogenarian yankee widow. Problem solved. But... these waters have sharks - what will Bill do now? Even though deep down the reader knows the hero will probably survive, make it doubtful. This creates suspense. Bill is a strong swimmer, but the pier is too steep sided to climb up, and the sharks are circling. Can he make it to the dinghy 50 yards away across open sea? Problems have to be solved in convincing ways too. If you already established that Bill isn't a strong swimmer, getting him to do the 100 yard crawl in 11 seconds to escape sharks isn't going to work for any reader. This 'crescendo' of plot development is particularly easy to organize using the 'Plot Cards' system at www.GetPlotted.com. Using cards allows you to lay out the plot in any order you like until it has exactly the right pace and feel to keep an audience entranced, without exhausting them from 'action overload'.