In contrast to other addictive substances, alcohol abuse leaves the person in question more understandable and with a little more clarity. Substances can wreak havoc on your mind on a near-constant basis, and alcoholism has peaks and valleys. Even the worst alcoholics can't drink every waking moment, so there will be times when they can understand what you're saying and doing during an intervention.
Anyone would be angry if they were "ambushed" when they realised they were in an intervention, but the immediate anger element can be removed if done correctly. An intervention does not have to set them off, but if it does, those launching it must remain calm and collected. Experiencing outrage, even if you're being screamed at or told extremely hurtful things, can be like cutting off a lifeline. At this point, it's not about you; it's about them, getting them help and getting through to them.