This is a loaded question in the comics and game community (and while I'm speaking directly to that community, I feel that this applies across the board to ALL creative endeavors).
With the rise of the internet, there is absolutely no shortage of artists looking for work. There is also no shortage of people wanting to have work done for free. And as a young artist, sometimes, it can be difficult to figure out just what the best course of action is as you build your portfolio and reputation. It's also easier to take chances when you are younger and fall into traps because you just don't have the experience to know what kind of mess you might be getting into.
There are several situations you may find yourself in -
Somebody approaches you to work on the project for a royalty. This is typically called spec work and no matter what the person approaching you might say, there is no guarantee of payout. Please be wary of dropping weeks and months into a project with no guarantee of payout.
The other situation you may find yourself in is the case of free samples. Again, this is often considered a test to potentially get a project. I'm here to tell you - any decent art director can take one look at your portfolio and know whether or not you'll fit the project. Plus, you may sink days or even a week into working on these free samples and still not get the job. Samples are fine, but getting some sort of payment for them is reasonable.
Finally, watch out for contests from big companies. Often, a large, well established company will run a contest for a poster, design, or anything, really. Please read the fine print - often-times there is just one reward and the company running the contest will own rights to every contest entry. This is simply an easy way for them to generate new ideas cheaply.
I personally feel that any payment, even a low payment, is better than no payment. There is almost always room for some sort of negotiation. And it's not good for the industry for artists to take on free work as it devalues the work across the entire industry. More personally, it's also not good for you, as I'm pretty sure you are still going to have to pay your bills on time, whether or you are paid or not.
Now, there is almost always an exception to the rule, so when you find yourself in this situation, and you will (every professional I know has been in this position), you need to ask yourself several questions -
Who am I working with? Is this somebody I know of? Do they have a reputation in the industry?
What is the product I'm working on? Is it well known? Does it have a following?
If the person approaching you cares so much about this project, why aren't they willing to put their own money into it?
Sometimes, you may find yourself in a situation where the lines aren't so clear. It's easy to turn down a project from a complete stranger. But if it's a peer or colleague, or even somebody who has some pull in the industry, it may be worth taking a chance. The same can be said for established properties. If it's an established brand with a large following, it might be worth taking a chance.
To give you several examples -
I took on a coloring project for a graphic novel a couple years ago. It was for a well established property with millions of novels sold and a feature film in production (which, incidentally, flopped). I researched it before hand to find all of this out and to educate myself on whether it was a smart business decision or not - from all indications, it seemed like a sure thing! I colored over a hundred pages of artwork over the course of a year and at the last minute, the owner of the property decided to change the terms of the contract, and I've never seen a dime. This is a cautionary tale - even with research and a well-established property, taking on spec work is still a gamble.
Several years ago I met an industry veteran, Bo Hampton. I'd grown up reading his books and I have always been a fan of his work. We started talking, exchanged some emails, and eventually, he had a really awesome project come together called 3 Devils. It's a supernatural western with zombies, werewolves, and gypsies and he invited me to work on it with him. For me, since Bo was somebody I'd grown up admiring and reading, I jumped at the chance to work with him. We'll be publishing the series through IDW later this year. In this case, the royalty didn't matter as much to me as the opportunity to work with an artist I admired.
You have to judge each project on a per case basis, just keep in mind - nothing is a sure thing.
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