But when you are doing an animated voice, it has to have more energy than usual or it falls flat and doesn't work. For myself, I found that I had to put myself in the same physical or emotional state as Sid, in order to make that voice sound alive and authentic. So if there was a scene in which he was running, I would be running beforehand to sound out of breath. That's important because the audience can tell intuitively if it does not sound real.