Before You Voice Act, Do This. All Of This.
There are some very gifted actors who can pick up a script sight-unseen and nail a performance on their first take. I've seen it happen. But the rest of us mere mortals might want to do a few extra things before we set foot anywhere near a studio.

Keep this handy list of cheat sheet tools somewhere close, and especially when you're starting out as a voice actor, refer to and perform each of these steps as you grow into the award-winning performer you will ultimately become.
a. Obviously, you’ll want to read each script first, if it’s not an in-studio cold read. Silently is fine.
b. Then, read it again. Maybe out loud this time.
c. And then out loud again. Now, you’re getting into it! Beginning to act out your part.
d. Record it! On a smartphone, video camera, or whatever.
e. And play it back and watch carefully. You’ll begin to hear things you missed as you were speaking.
f. Then, you can make adjustments and read it even better. It’s a process.
Many scripts will come with production notes that’ll help you understand the author or director’s intent. The age of your character and some hints into the personality. Perhaps when and where it takes place. Hopefully, a preference on how the character’s voice should sound. Warning - it might come across in a strange way you'd never expect: “A cross between Sponge Bob and Darth Vader.” What?
BOLDED or italicized words indicate a beat change – meaning you’ll need to adjust your tone, timing, volume, or attitude. By the end of the script, you’ll hopefully understand why these lines were written. What problem you solved. What promise you’re making. Why your character is supposed to do what it does. If you see something that indicates a change that wasn’t marked on the script, proactively mark it yourself on your script with a different color or highlight it to remind you.

If you’re opposite other characters, they probably won’t be in the same booth or studio as you are. Today, most voices are recorded individually. But you'll need to pretend and really believe they are right there next to you. Visualize them. Look towards them. Reflect on them. Pretend you’re actually interacting with them. That makes your performance sound believable. This is where that acting thing comes in.
Remember, your goal is to NEVER sound like you’re reading, although ironically that’s what you’re really doing. You need to be able to read without anyone knowing your reading. As you review your first few recordings, you’ll notice it sounds like you’re reading. As you develop more confidence and skills like cadence, you’ll begin to distance from that sound. Before you know it, you’ll be delivering Hollywood-worthy reads just like the pros. It’s not a quick process, and not everyone can achieve this, but it’s entirely possible.