Originally posted by dlh The simple answer is, "yes". It does matter whether you intend to sell the pictures. Though not necessarily to you.
Starting with the work in progress that is the subject of your picture. No question that the artist making the work owns the copyright to whatever he's done at the time of the photograph. What you'd be doing is "making a derivative work" as to that object of art, assuming that it's visible in the picture. You definitely need permission from the artist to do that, regardless of whether the picture's for your own use and enjoyment or for commercial use.
Secondly, and as to the picture of the artist himself and of his workspace: generally, you don't need permission to photograph people in public places or in places that you control at all. In some places, architecture can be treated as a "performance" of a copyrighted work, namely the plans and elevations of the building. The artist's workspace may be artfully arranged but that doesn't make it a copyrightable work unless he's developed a written design constituting a "fixed expression in a tangible medium".
Third: any commercial use of the picture requires permission, both for the derivative work as well as the image of the human. Commercial use doesn't mean selling a copy of the picture, the copyright, or a license, it means the use of the picture to sell something else. There was a famous case that started this line of thinking, in which a sweet, chaste, and lovely young woman (those adjectives were considered appropriate and relevant at the time she was alive) was photographed in public without her prior permission, who subsequently saw her image being used in advertisements for a brand of bread. She sued the advertiser and won, not because of the photograph, but of the use of the image of her likeness had been used for commercial purposes.
Fourth: ANY use of a copyrighted work without ownership or licensure ("license" being legalese for "permission") is prohibited, and that would include the work in progress on the artists bench.
Evidentiary issues:
The first observation is that the whole point of having evidence (e.g., a written contract) is to be prepared for the legal battle when things come to blows. Ideally, that will never happen, but the time to create such evidence is contemporaneously with events, well before you need it. People have come to me wanting to take action against someone without any evidence at all, thinking that they can generate what they need on the day of trial because they know the truth and they're confident that once they tell the truth, everyone will immediately see that their cause is just. Ooops.
Permission may be given orally or in writing. But the whole point of the exercise is to first, avoid legal problems at all, and secondly, to be prepared to mount an effective defense, just in case. You could document permission by turning on video-mode on the camera and recording the artist saying, "Yes, it's ok with me if you take all the pictures you want, and I don't care what you do with them.", though it's best to have a written memorial of the agreement. Some folks carry forms around with all sorts of legal mumbo-jumbo, but in my opinion, that's usually not necessary (for "walking around" type pictures). Arbitration and indemnification clauses only create confusion in my view.
The one thing I would include beyond the foregoing would be something that clarifies that the picture could potentially be used to sell products and services. I've seen some that say, "including commercial use", but the reason for that clarification is that some folks have been successful in arguing, "It's true I gave 'plenary permission', but I didn't know that included commercial uses. I only gave permission to take the picture, not to sell stuff." I defined "commercial uses" above because I think most folks wouldn't immediately understand the legal distinction between selling a print and using the picture to sell stuff. So to completely eliminate that argument, I'd make it clear if you intend to sell pictures on Shutterstock that such permission has been granted by a phrase that makes it clear, like, "...including the right to use the picture in advertising".
Note the distinctions between "selling" (conveying all right, title, and interest) and "licensing" (a grant of permission, which can be exclusive, i.e., excluding even the author, or nonexclusive, and which can be limited by other terms and conditions) and between the copyright in the work (an intangible ownership interest) and the work itself.
Then, there's the documentation of permission from the artist as to his copyrighted work - "Should the resultant images reflect one or more works in which you claim ownership of copyrights, you also grant plenary permission to make derivative works thereof for any and all uses, including advertising." or something like that ought to work.
Documentation may also required for subjects under a certain age where body parts are exposed - modern "child pornography" laws aren't as limited as older anti-obscenity statutes, and now include any depiction of practially the entire ventral surface of anyone under the age of eighteen as well as any part of the buttocks. The average bathing suit typically worn by teenagers would prohibit their being photographed, since some part of "the breast" will be exposed. And because the word, "breast" is not defined by statute, it may be used in the original denotative sense of the entire ventral surface from neck to navel, or it may be used as colloquially to mean "a single teat". One case from a U.S. Court of Appeals said that "the inner thigh" was part of "the pubic region" and found a photographer guilty of child porn because he'd taken pictures of kids on a beach. Here's a couple of kickers - any parents who sign a permission slip to photograph their kids in bathing suits will be guilty of conspiracy to commit child porn, because there is no such thing as "permission" in that context; and also, there are documentation requirements for pictures of anyone who so much as looks like they may be under eighteen - there has to be a statement accompanying such pictures telling the name and address of the "custodian of the records" proving that the subject is over the age of eighteen years. Because the statutes are written to favor prosecution of "thought crimes", the can be very widely interpreted - I've seen photos on this website that could well have resulted in prosecutions if anyone had a reason to want to "get" the photographer for, say, political reasons.
And, all that having been said, keep in mind that I'm not an attorney where you live, so this is mere top-of-the-head personal opinion and pontification about U.S. law stimulated by your hypothetical issue, not legal advice.
Let me know if I've left anything out, please.