A Word About Viola
Viola successfully passes herself off as a boy for the better part of five acts, and when her supposedly-lost brother turns up, everyone mistakes the one for the other, even when they are on stage at the same time.
This isn't what you might call exactly realistic, but it is pretty Shakespearean.
So . . .
If your character is encountering Viola while she's pretending to be Cesario (and she generally is), it will not be readily apparent from her voice, appearance, or mannerisms that she's not the boy she's pretending to be.
If your character is a mind-reader, or always knows the true nature of things, etc., or if they've read the play, then that's a different kettle of fish.
Use your best judgment, have fun, and feel free to leave a comment here or otherwise get in touch with me if you have questions, etc.
This isn't what you might call exactly realistic, but it is pretty Shakespearean.
So . . .
If your character is encountering Viola while she's pretending to be Cesario (and she generally is), it will not be readily apparent from her voice, appearance, or mannerisms that she's not the boy she's pretending to be.
If your character is a mind-reader, or always knows the true nature of things, etc., or if they've read the play, then that's a different kettle of fish.
Use your best judgment, have fun, and feel free to leave a comment here or otherwise get in touch with me if you have questions, etc.