I have chosen Caliban to discuss, since, as an actor, I find him the most interesting character and thus the most enjoyable to discuss.
Caliban's function in the plot is one that is difficult to define. He is not the key protagonist, since this title belongs to the treacherous Alonso in his usurpation of Prosporo. Infact he does not at all directly encourage the conclusion of the play.
Caliban has many small but essential functions; one of which is to create Shakespearean comic relief in his drunken trio with Trinculo and Stephano. He also creates contrasts with other characters, such as Caliban's association with the "earth" and evil magic (by being "got by the devil himself upon thy wicked dam" who is Sycorax, a which). This is contrasted with Ariel whose very name associates him with the air, and being a spirit he is also seen as a positive embodiment of the super-natural.
Caliban's lust for Miranda in "seeking to violate the honour" of her, is contrasted with Ferdinand's true love.
Miranda: Do you love me?
Ferdinand: ...I...do love, prize, honour you.
There are many suggestions in The Tempest' that give us clues into the character of Caliban such as being referred to continuously as a tortoise, fish, cat, monster and a misshapen knave, his very name has similarities to Cannibalism.
His mother being a witch does him no favours, but her treatment of Ariel (who we believe to be a "fine apparition" with his beautifully energetic language) certainly reflects badly on Caliban as a blood link, since she imprisoned Ariel in a "cloven pine...(for)...a dozen years". Then there is Caliban's attempt to "violate the honour of" Miranda; and at present not to be filled with guilt at this event but to say "would't had been done!...I had peopled else this island with Calibans". This certainly portrays Caliban as cold, evil and relentless that he would have repeated the rape.
Then when worshipping the drunken fool Stephano as a "God" and promising to show him "every fertile inch of the island", which is infact the same mistake he made with Prosporo, as he explains in Act one: "I showed thee all the qualities o' th' isle....Cursed be I that did so!". And now he makes the same mistake.
Then promising Stephano that Miranda "shall be thy bed" and asking him to "brain" Prosporo "and with a log batter his skull" it is clear there is certainly an evil side to Caliban.
He is also guilty of a credible proportion of the deadly sins:
Prosporo:There's...business for thee....
Caliban:I must eat my dinner.
He is thus guilty of gluttony and sloth, and in his attempt to "violate" Miranda he is guilty of lust. He also rather blasphemously worships Stephano's "liquor" as the bible.
However, their is a distinct otherside to Caliban in his soliloquy
"Be not afeared; the isle is full of noises,
sounds and sweet airs, that give delight, and hurt not."
It is certainly clear here that their is a more sensitive Caliban, not only in his concern for others in reassuring them that there is nothing to fear, but also in Shakespeare's alliteration of the s' sound to create a soft and sensual mood, and the use of words that portray a more emotional Caliban such as full, sweet, airs, give and delight. This emotional side also brings to light that Calibans hate for Miranda and Prosporo (ie, what portrays him as generally evil) is only to the fault of a building hatred by continual slavery and imprisonment. This is supported by a clear suggestion that before he was abused he also could appreciate emotion: "When thou cam'st first, thou strok'st me, and made much of me....And then I loved thee".
Never the less it is still essential for Caliban to be visually an animal, since he is described as "disproportioned in his manners as in his shape". Therefore I, playing Caliban, would twist my right foot inward at all times and always lean the majority of my weight on it. I would hunch my back slightly and keep my head lowered, and also continuously frown. I would also, when standing still, sway slightly from side to side, then when I need to move it would be a scrabbling furious movement to my destination, unless otherwise desired for the specific moment. All these traits and physicality's will portray Caliban as a non-human, animal-like creature. For the same reason I would also keep my voice generally low and harsh on the sounds. This will also meet one of Calibans plot-functions, by clearly contrasting Calibans sluggish movements with Ariels assumed flighty motion will clearly distinguish their metaphorical associations to Earth' and Air'.
I do however acknowledge that this will signal to a modern audience (that are naturally open to stereotypical thought) that Caliban is a messy, nasty, and an evil character (rather than a character that is a combination of good and evil, as in any real person). However, a modern audience are also able to change their opinions when there is a clear suggestion that he is not pure evil, such as his soliloquy.
In order to suggest this within his soliloquy, I would kneel (both knees) centre stage and become very motionless on "the isle is full of noises" and thereon, to turn the audiences attention to his internal rhythm. The only movements would be his mouth and his facial expressions, which would be a dreamlike smile and his eyes to close slowly. This would reflect the beauty of his words. By him being motionless with his body, it lets the words create the mood on their own, since the words are strong enough, but if we were to join them with elaborate body movements it would only distract from the spirituality of it. By this spiritual feeling the audience would be fully aware of a positive side to Caliban.
However, they would certainly not be aware of it in act one, where the conflict between Caliban, Miranda and Prosporo is overwhelming. This conflict is based on Calibans attempt to "violate" Miranda a few years previous and now Calibans punishment is pain.
Prosporo: "I'll rack thee with old cramps, fill all thy bones with aches, make thee roar, that beasts shall tremble at thy din".
Here I would be curled up on the floor in a ball, as if trying to protect myself from Prosporo's satanical words, I would moan in uneven cries to portray an animal-like fear in remembrance of this pain as previously caused. Prosporo would be looming over me, clearly, in terms of proxemics, giving him the power. Calibans fear of Prosporo's magic and the conflict between them would be clearly established.
The relationship between Miranda and Caliban can also be established in:
Caliban: "You taught me language; and my profit on't is, I know how to curse. The red plague rid you for learning me your language!".
He is quite obviously speaking to Miranda since she has just said "I...took pains to make thee speak", and now Caliban attempts to futilise Miranda's "pains" by displaying disrespect for the language she adores, in two ways. One, he only credits the language for its abilities to express his hate, ie "curse"; and two, by being grammatically incorrect by using "learning" and not teaching'. This is obviously intentional on Calibans part since he is clearly aware of the word "taught" in the previous sentence.
I would bring in this line directly after the previous line to display Calibans eager and passionate hate. I would yell "you" to establish his anger, and on "rid" I would thrust my hands towards her as if cursing her with the "red plague". This will make Caliban's contempt for Miranda evident to any audience member.
Not that Caliban holds contempt for all, infact it takes one swig of "celestial liquor" and Stephano becomes his "god". However, due to Trinculo's insults towards Caliban such as "a most ridiculous monster" he does not hold him so highly, so Caliban fights back with the following:
Caliban: (To Stephano) Let me lick thy shoe.
I'll not serve him; (Trinculo) he is not valiant".
I would be kneeling down at Stephano's feet, again giving him the power, and be looking up at him with glazed admiration in my eyes and a cheerful smile (due to the intoxication's). Then rather abruptly wish to display my animal affection by offering to "lick thy shoe" -eager to please. Then bend my head to do so, but then decide, while under the protection of Stephano (who is blinded by his new found importance) I would stop suddenly and turn slyly to Trinculo and say "I'll not serve him; he is not valiant" with a rising of the eyes and a sly smile on "he". This will portray his admiration of Stephano and his dislike of Trinculo. This will also meet another of Calibans plot-functions, by the obvious comedy (ie. drunkards, drunkard being perceived as royalty, Caliban's ignorance and also how low he will stoop etc...) acting as Shakespearean comic-relief.
Therefore, this is how I would create Caliban on stage, considering his function in the plot and his relationship to other characters.