Seriously, I have a very simple tip for you the aspiring actor, artist, engineer, alchemist of yourself. How to totally nail a role in a movie, play, or commercial.
It's not complicated at all. The secret is simple. Have you heard the expression "stepping into their shoes?" That is the answer. You LITERALLY must step into that character's shoes and become him or her on the outside before totally nailing the internal state or disposition.
In other words, dress like the stereotype of that person would indicate. For instance, if you are auditioning for a role as a cowboy, guess what? You need a pair of boots and a hat at the very least. I would recommend going all out and getting a belt with the most ridiculously large buckle. Now you're talking, partner.
Of course there might be roles that are a bit more complex and ambiguous in that you cannot easily find some stereotypical type of wardrobe for him or her. In that case, find something that you think would make that person unique and make him stand out and that would serve as a good substitution for a clearly defined typical wardrobe. Maybe it could be a nervous tick, or some compulsive idiosyncrasy like washing his or her hands frequently starting from the thumb on the left hand and going one by one until you finish on the pinky on the right hand EVERY SINGLE TIME>>>. Now that's really getting "into" character.
But by far, I think that the easiest way to get into the role and nailing the character without fail is wearing the clothes that he or she would wear. That gets you into that mind frame. Then after consciously doing that many times over and over you will naturally feel like that person and won't be tied to the clothes because having started constructing the character from the outside, you will inevitably end up inside at the core BEING the character. As the saying goes: "clothes make the man..."