Michael K Williams cannot whistle! It’s totally true. We brought him in and he tried but it just wasn’t happening. Omar’s whistle is provided by a lovely and talented loop group member named Susan, who is an actor and John Waters’ personal assistant.
From a Reddit AMA with a sound editor from The Wire, I think I’ve read some of it before actually (though I missed the Omar bombshell).
The description of Dominic West’s ADR (looping) sessions is wonderful:
…a Dom West ADR session often went like this:
Me (with Dom staring at my mouth): Cunt. Cunt. Cunt.
Dom: Cahnt. Shit, do it again, please.
Me: Cunt. Cunt. Cunt.
Dom: Cunt. Cunt. OK, let’s record…
(three beeps, the line starts and):
Dom: …cahnt. Feck! Say it again.