One of the major themes in Twelfth Night is Viola’s cross-dressing in Act I, Scene V. Although there is no direct explanation to why she disguises herself as a man, there are a few assumptions we can make that make sense within the context of the play. One reason may be that by presenting herself as a man (Cesario), Viola is able to speak with Olivia in such a way that would be unacceptable if she were a woman. Also, because “Cesario” is viewed as a young teenager, or as Malvolio puts it, “Not yet old enough for a man, nor young enough for a boy” (1.5, 143 – 144), he may be able to get away with speaking with a flair of wickedness because he is still too young to be taken seriously, therefore too young to be reprimanded for speaking to a woman with a caustic tongue.
Because Viola had just arrived in Illyria, no one knows her yet, so no one has any doubts that “Cesario” may not be who he says he is. This anonymity could very well be in Viola’s favor because while “Cesario” can beat around the bush with Olivia to see how she reacts to the disguise, Viola will not be blamed for “Cesario’s” rude behavior. Viola’s disguise can also act as a protection against her clean slate; if Viola, not in disguise, were to say something rude to Olivia, her good name can be ruined and will be branded as a rude woman. But for Viola to disguise herself as a teenage boy named Cesario who doesn’t actually exist, “Cesario” will never be seen again anyway. It’s the same concept as if Viola put on a costume and slapped Olivia in the face, only to run away and come back as herself, allowing herself to get away with her mischief.
Another reason for Viola’s cross-dressing could possibly be to cope with the death of her brother. Viola may be borrowing personality traits from her brother, who was notably courageous, especially in his efforts to save himself from drowning as the captain mentioned in Act I, Scene 2, ” I saw your brother, Most provident in peril, bind himself, Courage and hope both teaching him the practice, To a strong mast that lived upon the sea” (1.2, 10 -14). Viola is being courageous herself by actually going through with the plan to cross-dress, even though there is a chance that someone could (literally) see through her disguise.