Sometimes I hear an idea and my initial reaction is "No," when what is really going on inside me is "Let me mull it over." The trick for me is to not open my mouth too soon before I've truly given the idea a chance to breathe a little.
Even if my final answer is negative, the fact that I took some time to consider it is perceived as very positive by coworkers and managers.
So I see someone who says something I don't entirely agree with but I want to see where they are coming from. I start arguing with them, asking the harder questions, pointing out things I see as problems, not to push back or prove that I am right, but to prove the merit of their ideas. Sometimes I will say something to indicate why I am doing it, but for people who know me, I usually skip that part.
The goal is then to argue, hash things out, force people to defend their ideas, and then what's left after that session can be assumed to be better than any of us could have come up with otherwise and we go forward on it.
However, there are two secrets to making this work:
1) You have to be known for actually using the ideas that are effectively defended. People need to know it's not personal and that it is both professional and empowering to them. If I argue with your idea, it is because I see potential value in it but I want to make sure it doesn't easily collapse. In some cases I will offer to stand aside remaining objections or in some cases, I will endorse the idea.
2) You have to be open to (and appreciate) everyone else doing the same to you. The point here is that it is a vetting process, and my ideas can be challenged just like everyone else's. In fact if you challenge my ideas, that's again a sign of respect. If you didn't value the ideas, you'd probably ignore them or (if they go against your stakes in the project) argue instead that they are dangerous or problematic.
Being argumentative, like being defiant, is a powerful tool, used right. It just takes some time and effort to make sure that it is used right instead of being used in a way that is destructive.
However, you can also wrap that in a friendly approach as youlost_thegame mentions - simply acknowledge the potential of the idea first (ie, positive energy) then go in with the tough questions.
I find even with friends and close colleagues, if I stick to the "nice first, then difficult questions combined with enthusiasm", I can often critique the idea while making the entire conversation positive. Often the faulty idea gets discarded by it's owner after consideration.
1) Those who know what's going on and treat it as a sign of respect.
2) Those who don't and where you have to show externally that there is respect involved.
For the first group, getting into an argument on technical merits doesn't have to involve a whole lot of pleasantries. It's largely informal. I try to save this for groups I am a long-standing member of.
For the second group, you are right, a softer approach is necessary.
You know, explaining an idea to somebody, from scratch, it's the better way to develop it. If it's stupid they will realize. If it's good they'll get enthusiastic.
In a previous comment I mentioned that this is the tactic that my manager uses, and it works wonderfully: let people talk from the beginning and ask questions later.
Of course, if you end up disagreeing, even though if their idea is good, they'll like that you let them explain and took time to consider
Part of the use case of a "trial by fire" approach is that the idea is to come up with a better approach than any one of us could ideally. One isn't interested in trying to use their idea as-is, nor in simply disregarding it, but in probing, understanding, and adapting it to the case at hand.
That means arguing about it in detail. Explaining from scratch may be a necessary prelude but it doesn't help really with the final deliberative process. The key is to ensure that everyone knows that these are good-natured and that most people will shape the outcome even if their ideas are not adopted wholesale.
The goal is to have an argument where everyone collectively wins, and where each reasonable participant (meaning, most of the time, every individual participant) individually wins as well, in the sense of having their concerns incorporated into the final view.
In fact, I have one colleague who immediately says "no" to every one of my ideas before almost exactly one month later saying yes emphatically (and then going on to claim it as his own idea all along). It's uncanny - I have worked with him so long now that I can almost time it to the day when he will agree with my idea and claim it as his own idea all along. It used to really bother me, but not anymore - as long we get things done I don't care who "owns" the idea.
Silence is the ultimate wisdom. I just proved I'm a fool by even mentioning it.
(If you had kept your silence, you would have stayed a philosopher)