Some other writers go even farther and suggest reading your entire novel out loud. I would agree (just don't do all at once for the sake of your vocal cords!), but I would take it one step further.
Act your novel out. Yes, act it out. Stand up, find a mirror, preferably in a bathroom because your voice will be amplified, and pick a favorite scene. Get rid of your self-doubt and embarrassment heebie-jeebies (no one is watching!) and channel your inner main character, antagonist, love interest, and walk-on.
There are a number of reasons this system is helpful (and fun!).
1. You will get a better grip on who your characters are
If you find yourself merely standing there reading your dialogue aloud, you have a problem. In reality, people have accents, mannerisms, distinct expressions that their friends recognize, and these things can convey personality without you needing to know their backstory. I admit that it's very difficult to convey mannerisms through the page, especially through first person narrative, but this, my friends, is what makes your characters come to life.
These same mannerisms and habitual quirks are what actors use to define their character's part, and animators for cartoon people's personalities. I'm pretty sure, after seeing The Hobbit and leaving the theater with my friends, that we were all doing Martin Freeman's Bilbo nose wiggle. Just that little tick alone brought something more to Bilbo's personality, and made him feel more alive to us, the audience. You should do the same for your own characters.
Another way mannerisms are crucial is that they portray emotion through showing instead of telling. They cut out the need for unwanted adverbs. For example:
John turned towards the window wearily. "It's been such a long day," he said, sounding tired.
John turned towards the window, pinching the bridge of his nose. "It's been such a long day," he said with a heavy sigh.
The latter sentence, which shows John's exhaustion through his body language, avoids simply telling the reader how he feels.
Find mannerisms for your characters. Don't overuse them, but do take advantage of them. And the best way to discover new ones and ways to describe them is to act them out yourself.
2. You will have an easier time with combat and dance scenes
If you write fantasy, and if this fantasy is set in any period prior to the 1500s, then I'm going to assume you have some pivotal dueling scene. Acting out your duel can be very helpful for understanding the psychologically grueling reality of broadsword dueling (which I may also touch on in another post), and will help you decide what movement to write into the narrative and what parts to gloss over.
The same is true for dance sequences. I laugh at some historical-set stories sometimes because the author clearly did not understand the sort of dancing that would have existed in their era of choice and write these long conversations between characters a la Darcy and Lizzie. While their day's dance would've permitted such a thing, many others, such as those in the colonial period, would leave characters long out of breath by the end of the scene!
And of course, this applies to any situation where there is lots of physical exertion. Sports, riding, games, etc. I'm not suggesting you act out the entire duel/dance/sports play (that would get old fast!), but go through some of the motions. Get a sense of sympathy for your character's situation. In order to create believability, you can't just research these things. You have to immerse yourself in them, and acting portions out will assist with this.
3. You will get a feel for the flow of the scene
It isn't enough for dialogue to sound natural. The entire conversation, and what happens in the pauses
between remarks, must be just as natural. This is important for scenes with lots of emotion. It's easy to get carried away in the moment when jotting it down, and end up with a disjointed sequence.
Now, I am a plotter. The scenes I write I have planned meticulously for a long time. But I'm more than a plotter. I'm one of those people who try to write the best first draft that they are able. But that's just me. I know I'm in a minority, but as a former pantser (someone who simply sits down and writes without too much pre-planning), I can testify that sometimes, acting out the scene you're about to write can be extremely helpful for pantsers, as (in my short experience as one, anyway) they get writer's block frequently. When you aren't sure what to jot down, get up, and let your characters direct your movements. You may be surprised at what comes out. I used this method when pantser-writing my YA sci-fi a couple years ago, and it helped me break through writer's block plenty of times. And even as a plotter, I still do. In fact, occasionally my CP and I will act out scenes together. It's a whole lot of fun and enlightening at once!
In conclusion, many of the writers I know in person all share with me an ability to mentally play their story before their eyes like a movie. If this is the case, you should have no trouble acting out your scenes. In fact, it may make your "movie" more solid in your mind, and therefore your writing more solid on the page. Try it! You may be surprised.