The theme of deception is an important component of Twelfth Night. Physical disguises, forged documents, and blatant lies allow the play to think about the relationship between appearances and reality. In the case of physical disguises, costume, voice, and demeanor all forge one's social identity, but don't necessarily reveal one's inner nature. Words (both spoken and written) are also associated with deception, and leave characters vulnerable to trickery, especially when gullible figures already suffer from self-delusion.
Twelfth Night is concerned with the possibility that outside appearances do not always match or align with inner character.
In Twelfth Night, outward identity is not the only thing that can be disguised – written letters and professions of love can also be concealed and associated with deception.