Think about it. In the first scene of the third act, Feste really seems (at least, to me) to be flirting it up with the disguised Viola.
He compares words to whores and drops a ton of hints that he knows that Viola is actually a woman. Feste also says at one point, "... I do care for something. But in my conscience, sir, I do not care for you; if that be to care for nothing, sir...".
In that couple of lines, Feste offers more of his real personality and view of reality through his honest interactions with Viola than with any of the other characters in the play.
Also, after Viola pays him a coin, the fool goes up to her and says,'...would that I bring a Cressida to this Troilus". Really, Feste? Troilus and Cressida? They're basically the Greek version of Romeo and Juliet. If that's not a classy classical pick-up line, I don't know what is.
And besides, in every production I have ever seen of "Twelfth Night", the beginning of Act 3, Scene 1 is always interpreted as Feste falling in love with Viola, or flirting it up.
In the most recent production I've seen they downright had Feste falling head over heels for Viola and him covering her ass from the craziness in the final act. He even gave Viola a kiss at the end. It was very bittersweet and emotionally powerful.
Maybe I'm reading to much in between the lines, but I'm only commenting on what I've continuously seen. Make of it what you will.
