Pet Peeve: people who don't think things through

A huge pet peeve of mine is people that don't think through things. For instance, there's a guy at my work who wants to implement a new idea. However, when asked questions about his idea, he can't answer, can't explain the idea, and gets defensive. So it doesn't lead to a lot of buy-in on my part, ya know? Who are these people that think "oh, i have an idea, i'll just implement it without thinking through any of the details???" I don't get it. He wants to implement the idea and figure out the details later! He just expects us to take his word that he knows what he's talking about and go with it. The kicker--- he's been with the company for only 2 weeks! Who does this??? At two weeks, you still have to prove yourself and your ideas; I had to do that with my idea. I guess he's decided he's above that. This guy is so irritating. I never know how to deal with irrational people. I'm happy to say that I work for a company now that won't let this behavior fly.