Interventions are more about the people around the A than the A. They need to be as ready to make big changes as they are asking the A to be.
Empty threats will likely just make the situation worse. An intervention will fail - either in the short- or long-term - if it's used as a tactic to try to scare someone sober. And a bad/not-very-well-thought-out intervention (like the one my mom's sisters tried on her) will just cause major resentments from everyone.
Heh, yeah. Interventions didn't work for my STBXAH because we weren't on the same page about what the intervention was for. His family thought it was akin to "calling him out" and tried to coerce him into "telling the truth" about whether or not he was using. There was one time, for example, where we suspected he was using and got all worked up to intervene, and when we asked him about his use and tried to get him back to rehab, he said, "Oh, I'm not using." We all kind of looked at each other and hemmed and hawed, then his ex-cop dad who clings to denial like it's a barnacle on a whale was like, "Okay then, son," and we all went on our separate ways. But of course he was using!
We missed the point. It wasn't about him. The intervention was supposed to be about what we were willing to do -- or not -- for him in his disease. The intention is to "bring up his bottom," so that instead of having the slow fade of withered trust by individuals or the slow fade of opportunity and cash, the addict feels a hard crash of consequences when all his doors close at once. Ideally, with no one left to use, he will realize the error of his ways and get clean.
But there are some big caveats to whether this will work. Ideally all the family members are in a program and know what they're talking about and why they're doing this. No magical thinking here -- and no lingering sick family systems. If nothing changes, nothing changes. And another thing? There's always someone willing to be used. Even if the whole family does everything right, there's a good chance the addict has a friend or acquaintance willing to enable him further.