Purchase a basic theatrical makeup kit and experiment. If you're playing a sickly character, try blending darker shades under your eyes and cheekbones. This will help make you look gaunt and tired. If you're playing a childlike character, blend some blush onto the apples of your cheeks and maybe color in some freckles. You can also purchase fake facial hair and noses. Nicole Kidman wore a fake nose throughout the shooting of the film "The Hours," for which she won an Academy Award. Try altering your features to suit your character. Countless online tutorials will guide you for free.
Go to a wig shop and find an appropriate wig for your character. To make the disguise really effective, try finding a style that looks nothing like your own hair. If you have long hair, try stuffing it into a very short wig, for example. You could also use your own hair to create a style that looks completely different. Using hairspray and bobby pins, actresses can create some wild and different looks for their disguises. Men can create a slick look by putting gel in their hair and combing it back.
Change your voice for your disguise, so that you don't sound like yourself. If you are playing a damsel in distress, maybe your voice goes up an octave to speak at a small, high pitch. If you are playing the villain, perhaps you have a snarl or an evil laugh. Changing the placement of your voice is very effective as well. You can move your voice from a very nasal sound to a chesty sound simply by concentrating your energy in different places and feeling the vibrations. Experiment to find your character's voice.
Choose clothing that your character would wear. While wearing the mask and wig, try on different outfits to see if they look right. Notice how the clothing makes you move differently. If your character is a glutton, try stuffing cotton into your costume for the appearance of extra weight. Women working on Elizabethan characters will find corsets very helpful for characterizations. A corset creates a different silhouette and forces the actress's posture upright. SImilarly, men often wore heels in Elizabethan times. Experiment with different shoes and accessories.
Change the way you move and walk around the stage. This is the final step that will transform you into a different character. Let the makeup and the clothing inform your movements. Use your imagination, and trust your gut. This is your disguise.