It is said that the philosophy on human nature is that we ought to be ourselves and not copy. This tends to make everything straightforward and somewhat real, as it causes less confusion between the sexes. Characters and language in this play can support this.
In Act 1 Scene 1 Orsino says "If music be the food of love, play on;
give me excess of it"
It seems that Orsino in the play has "fallen in love" with Olivia .Yet we as the audience understand that he has not ‘Fallen in Love’ as such, but the idea of falling in love is better suited to his belief, But not being able to admit this to himself he ultimately falls in love with the countess’s daughter. Orisno says and I quote" Give me excess of it" thus showing his need for love, almost like a drug, he will do anything for it, with this it shows a change in Orsino's true character.
The plot eventually thickens and Orsino’s love is rejected by Olivia yet he doesn’t give up, he continues to try, using Viola/Cesario as his messenger and his intent that this will get her to fall in love with him.
This is the set for something, something which gives the play that twist in the middle of it. Cesario's continued attempts to get Olivia to fall in love with Orsino, ends up in Olivia falling in love with Cesario, who as we know is actually a man which makes this a odd situation which is funny to see Cesario manages to escape from Olivia yet she can not keep her hands off Cesario.
In act 3 scene 1 Viola says "I am not what I am"
Viola in the play is recognised physically as a man as she dresses up to make a living in a "Mans" world where the majority of the male sex get an easier role in life as jobs for them are easy to acquire. A woman was supported by her man so the saying said, but Viola had no family or man to help support her.
Accepting this role is only the first step as it’s just the appearance that changes which is the most obvious. Now Viola is known as Cesario and then eventually, she mentally becomes Cesario and forgets about her original status as a woman, as Viola, becoming the character which was just to get a job and live a simple life. For this quote shows that although other characters don’t know about her disguise, she ultimately does admit to herself, showing self denial when disguising one’s self.
In act 2 scene 5 Malvolio says "To be Count Malvolio!"
While walking through the garden outside of Olivia’s house Malvolio makes this quotes, from this quote we understand that he imagines himself as the husband to Olivia and the title given to him afterwards, that title being "Count"
If not quoting himself Malvolio imagines himself as a high ranked person to those below him because of his jealousy towards Sir Toby and others because of their relationship towards Olivia, which as we know is closer, giving the audience a mean and evil sense and point of view of Malvolio, In this scene Sir Toby and Sir Andrew are behind the bushes listening to Malvolio talk to himself which is a comedy scene.
In the play, Malvolio is tricked where he is lead to believe that Olivia the countess is in love with him. This leads to him making a complete fool of himself in front of Olivia this makes people think he’s mad and eventually sent elsewhere.
Act 4 scene 2 we see Feste, whose disguise is well thought. We know he is the jester to Olivia, we find hum intriguing because of the way he is able to manipulate people. In this scene he takes on a role of a difference, meaning more physical. He is brought into the plan of mocking Malvolio who is in the dark house, he does this for Maria and Sir Toby’s enjoyment.
"Feste: Sir Topas the curate, who comes to visit Malvolio the lunatic."
Malvolio being as dumb as he looks believes that this is Sir Topas yet it is actually Feste disguised as him.
He plays along with Malvolio pretending to be on his side and Malvolio is open to him yet Feste/Sir Topas ends with
"Nay, I'll ne'er believe a madman till I see his brains."
This means he will never speak to Malvolio again until he is dead and then he will believe he is not mad.
So in Twelfth Night the Theme of Disguise is taught through the philosophy of human nature.