In what ways are Viola and Feste similar in "Twelfth Night"?

It's an interesting question. Both characters are often played as knowing rather more than they let on: Viola, of course, knows that she is really a boy, and holds the secret not only of her own disguise, but of her own love for Orsino, as well as the absurdity of Olivia's love for her.

Feste seems almost nomadic, absent from Olivia's house for huge stretches, but with unusual licence for a licensed fool (have a look at what he says to her when she first enters) - and of course, he also sings on occasion for Orsino. Although he doesn't say a huge amount, he sees a lot. And he's often played, particularly justified by his final song, as extremely wise.

So where does that leave us? Both of them occupy a position somewhere between Olivia and Orsino's households. Both of them have excellent senses of humour. Both of them take unusual licence with their respective masters.

And, some productions might argue, both of them are largely "unknown": Viola's real name is not mentioned in the play until the last act, and she hangs somewhere in the balance between male and female. And Feste says so little, and gives away nothing about his own background (his first scene has Maria asking him - with no response - where he's been) that he too is something of a mystery.