As a voice talent, you don't have time to develop the character in the script as you read along. If you're reading a shorter commercial script, you could only have 15 - 30 seconds. For a longer narration piece, you definitely have more time to flesh out your character, but you still have to start out knowing some background information. Regardless of the length you're working with, you need to know who your character is, what the point of the script is, and who the demographic is your client would like to target. What's the Deal, What's the Story?
Whether you ascribe to method acting or not when it comes to voice overs, asking yourself what was going on the moment before the script begins can be a valuable tool to helping you get into character. Short scripts usually fall into two camps:
1) Your character is an archetype who represents with a specific complaint, situation or idea. If the specific script is in your niche, you probably know what to do with it.
2. The script is so short that you're not sure if it's in your niche or not! Sometimes the copywriters aren't able to flesh out the script well enough, so they hope the voice actor will know how to interpret the copy.
In the second scenario, imagining a back story can help you get into character and sound more natural.
Choose a setting for the script that makes sense, and from there figure out what the character must have been doing right before the script starts. Who is the character talking to? Pick a situation and stick to it. Match your voice-over technique to fit the script; meaning, your pitch, energy, volume and attitude should make sense for the given situation.
Keep in mind that when you play it back, you may find your interpretation didn't actually make sense at all. That's OK! Tweak the back story and your character, but make sure to be specific with your interpretation. If you don't commit to a firm story or character, it will never sound quite right.