There was once a perfectly succinct article on this topic but I can't find it - so instead you get this one by me. Strap in, it's going to get a bit sweary - like all of my best writing.
This particular topic comes up every now and then - sometimes it even includes a request to sign an NDA.
Let's start off with a simple statement: I do not want your game idea. I do not want to steal it.
I've never really heard a single game idea that actually conveys something I'd bother to do myself. This isn't to say I've heard only bad ideas - more that every idea someone's pushed to me has felt distinctly 'not mine'. Part of this is also the 'it sounded cooler in your head' thing - game ideas don't really cover the actual feeling it's generating for the person who thought it up. I've been told some pretty ordinary ideas that people've made into extraordinary games.
The next point, is that I have my own shit to work on. Things that I personally feel are great (and if I told them to you, you'd be very point 1 about them). If you ever saw some of my older posts? Scred is going to be a very multi-entry franchise, and Arnthak pretty sizeable itself. This isn't even counting all the non-franchise stuff I want to get around to, or Xenoporn Tycoon (card game for later - notice how I mentioned it without being worried you'll steal my idea of a game about making alien porn?). With this in mind, why would I work on something you mentioned to me once in conversation? My shit's pretty rad already without touching yours.
As it is in your head, your idea is worth $-10. If you ever want to talk about flooded markets, the game idea market? Holy shit. There are bots that generate game ideas, and there's a whole subreddit of people hoping to get their game idea made. People try to tell me shit at parties (though I don't seem to go to that many parties lately). I find it incredibly hilarious that for a group of people who are continually harassed about making other people's 'great idea for an app/game', they seem to think their own are somehow different/rare. Ideas/concepts are worth nothing until they are executed - then they can be shown in their full glory.
This brings along the next point. When people think 'My game idea might be stolen', they probably are confusing this situation with 'my game got cloned'. This does happen. Quite often. See the whole 'Threes vs 2048' situation, or approximately 90% of the app store (hyperbole - but even still, how many flappy clones are out there?). I've yet to hear a single case of actual industrial espionage style 'idea theft', whereas there's essentially an industry of small studios ripping off the latest hits. See, the people making clones? They're smart. They don't give a fuck about your idea, they wait until they know it works out properly. If I were interested in stealing your thunder, don't you think I'd wait for it to begin raining (money) first?
It's also reasonably safe to tell someone your idea because they would (if you threatened to hurt them unless they went through with it) end up with a different game than yours anyway. Seriously. Most people describe their game as 'popular game X meets popular game Y', 'X setting', or by virtue of a singular mechanic. Have a look at the fact that gamejams tend to have a singular theme and then the variety produced from that is significantly large. You could tell me about your squirrel racing game down to controls, but I might make it voxels with a betting mechanic - while you might make it in ascii and first person POV.
Game ideas change and shift over development anyway. It's super rare to have a game that doesn't undergo some form of scope creep or revision. Arnthak went from being a 5 level single screen platformer about an evil mage reclaiming his tower to a sweeping epic with a super huge map and questing. (Not saying this level of scope creep is good, just saying it happens). Your idea on day X will not be the same as day X+200 - even if it just undergoes subtle alterations, so if someone were to steal it on day X, you'd end up with an entirely different game by release.
Now, allow me to strawman a response here: 'Yes, but what about Vox/Cubeworld' (or two other super similar games that came out the same time of your choice). This is pretty simple, and we have a nice term for it: Convergent Evolution. It's pretty great, and in terms of this discussion means 'your idea is not necessarily JUST yours alone - anyone else could think of it'. Notably common during times when there's specific market pressures (eg: who'd have thought two games with voxel-esque graphics and sandbox elements would pop up around the time when these two genres/styles were popular!)
Finally - attempting to keep your game idea secret leads to something I inherently dislike: developing in a bubble. If you're so scared of someone stealing your game idea, chances are that you won't ever send it out for feedback. You won't talk about it, can't glory in your successes with your colleagues. You can't write about it either. Gotta just keep working away in secret. What a horrible way to develop.
Down to the harsh end of things - most people won't even finish their game. Why worry about having your idea stolen if your project is probably going to fail? (If you think I'm being super down, have a look at the subscriber count of r/gamedev and then have a think about how many games are actually coming out of that subreddit).
Damn. I really didn't swear nearly as much as I fucking thought I would for this one. (oh, there we go. Yay!)