When working with a short commercial script or even reading a long narration piece, you don't have time as a voice talent to "develop" the character as you go along. Part of your pre-recording practice involves breaking down the script to figure out things, like: Who is your character? What is the point of the script? Who is the demographic you're trying to reach?
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 or vague that you're not sure what to do with it! Sometimes copywriters don't flesh out their vision as well as they think do, and you're left wondering what they're looking for.
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.
If you don't get it right the first time, don't worry about it! You should realize it early on when you play it back that your interpretation is off. Think of an alternate back story and try it again. Part of the fun of doing voice jobs is getting to experiment with different interpretations. It's even a good idea to send the client a couple different versions if you don't understand from the script what they're looking for. But remember: if you don't commit to a firm story or character, it won't sound right.