Masks are done. They don't look as terrifying as the inspiration (pictured below), but they're not bad for scary.
These madmen only appear in one scene and their lines are almost-but-not-quite rational. Webster offers us a mad lawyer, a mad priest, a mad astrologer and a mad doctor and gives them all lines explaining why they've ended up the way they have. Which is great. Huge fun to play and so on. However, every single one of them is also playing someone else as well.
Now in an ideal world, what we'd do would be workshop our way through using different vocals and physicality to emphasise the differences - and we will use those as well - but time is short. Easier by far to use masks. The voices will all sound slightly different behind the cloth and the movement issue is solved because these things act like blinkers and force you to turn to look at the person you're talking to rather carefully.
That's the practical reasoning. The artistic/aesthetic reasoning is that back in the sixteenth/seventeenth century, mental illness was treated with a dazzling mixture of fear and contempt. Masks immediately dehumanise the performers and also gives us a nice visual seed that can come into play a little later.
With sweet irony, Ferdinand brings the madmen in to send the Duchess insane but ends up going mad himself. He spends the last third of the play convinced he's a werewolf. Yes, yes, I know, but that's Jacobean Revenge drama for you. There is a scene late on in which Ferdinand is confronted by a doctor and has a similar mask dropped over his head. Being Ferdinand, he immediately rips the thing off and attempts to tear out the doctor's throat with his teeth, but it's a nice visual to bring up.