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) The character is easy to relate to and the problem or situation tends to be boiler-plate. If these voice jobs are your niche, you usually 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.
Figure out where the script takes place, what your character was doing right before the script picks up, and who he/she is talking to. Pick a scenario and commit to it. Then match your pitch, attitude, volume and energy to fit that situation. Your performance will probably become very natural after that.
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.