Love the premise, but one thing would make it much better: Show the cast preview in context (e.g. rendered as if posted in the reply thread).
Yeah, Twitter does this, doesn't it? The chance to undo is in context, not a "preview" screen. The preview screens makes casting seem more important/critical/risky than it actually is (for me at least)
agree that would be a nice touch, just a matter of priority Twitter doesn't do this today fwiw