"I have an idea. What if we did this?"
Sure, it's a start. But it's also too easy. Proposing an idea to your group just to put it out there and see what happens isn't much of a risk. The group often won't share your excitement. Or they'll poke holes in the idea and tell you why it won't work. And when the group doesn't jump on board, you're left with just an idea, one that nobody else seems to care about.
Doers just bypass the proposal.
"Here's what I'm going to try. Who'd like to join me?"
That's a lot different. No asking for permission. No debating or hole-poking. Just the initiative to actually do something and an invitation to bring anyone else along.
Doers tend to get a lot more done than proposers.