We're often consulted - and yes, it varies from ones where I (or others doing the same job) have complete control, through ones where we're consulted, to ones where we do exactly what we're told.

The most common is that the curator has a plan (sometimes drawn out in 2D/3D) of roughly (or maybe precisely) what they want, and where they want it, and we try and make that happen. We're often consulted and free to make suggestions that they may or may not consider (including stylistic, aesthetic and narrative ones), and we try and spot potential problems with the current layout - there are often considerations of lighting, for example - a projection wants it as dark as possible, the painting wants to be well lit - maybe don't put the projection next to the painting - you also get different types of work or ages of work which have different recommended (or maximum) lux levels of light (to avoid damage). For example, a historical paper-based work, might have a maximum of 40 - 50 Lux. A historical painting might want to be about 100 Lux, a contemporary one 200 Lux, a sculpture might be happy with 300 Lux. A low-value contemporary work, where conservation is not a priority, you just "light it until it looks good and bright". Obviously, light can spill from one work to the next - so we may advise to adjust the order or spacing to avoid this.

Essentially, the curator or gallery manager has final say - they are often the direct client/point of contact. They can insist we do something "wrong" (i.e. fix something in an unsecure way, or light something bright enough to cause damage) and we may do it, as long as they understand they have insisted it, and we have advised against it. In a sense, we can overrule a curator on health & safety, practicality, possibility etc - though ultimately they're in charge.

In Star Trek terms, the curator is the captain, and the art technicians are either medical, engineering or security. We can tell them "the walls won't take it", they can insist we try it anyway, we can quickly invent some techno-babble about "creating a support lattice behind the original wall allowing us to treble the possible weight it can take", then we give a time estimate (and we pad out the time estimate by a few hours), they say "make it so", we do it, and finish early, then they say "you're a miracle worker", which feels great.

As for what places have more or less freedom, it's often down to the staffing and "officialness" - a converted shop unit, for a temporary show organised by local artists, they might completely hand decisions over to me, or in heavy conversation at least. A college or university might accept a touring exhibition, and have an opinion on the order the works go in, or which ones should be grouped together, but the actual decision of what works on what walls, and the spacing etc is either worked out collaboratively, or left to us. A municipal art gallery, or larger arts organisation, may have a director, a chief curator, a gallery manager, an assistant curator, a registrar, a gallery assistant etc and in those cases, if they've got the people, they use them. Sometimes we're just hands to do what we're told. In some way, the higher the value of the work (or fame level of the artist/venue) the less control we have personally. We will often say, politely, if something is a bit... "Are you sure you want it done like this?" - but if they insist, well, we'll do it.

Most places will give us specific control of the spacing. A test layout of an exhibition is normally done by leaning the works up against the walls, stood on little foam blocks, then sliding them left and right until it looks okay. Quite often we move them to where we think they should likely go, and the curator would then say "a bit closer/a bit further apart" or "group those ones close and leave a larger gap around that one", for example. Once agreed it looks pretty much okay, then they'll sort of say "you rationalise the numbers", so we will be left to do the specific maths of something like "50cm from the corner, then 30cm gap between works, except these two are a pair, so they'll go 10cm between works".

Because we, as freelancers, work for multiple different venues, we're often consulted on "how are the other places doing it?", in questions regarding hanging heights, spacing etc. Nobody wants to be seen to be "doing it wrong", so in a way, we are sort of gatekeepers of what the average is. This is both good and bad, as I guess it keeps standards high across the board, solutions found in one place will spread to others in neighbouring areas (like technology spread in Crusader Kings), and places can trust that if they lend their works to another venue, they will be looked after properly. On the other hand, over time this probably leads towards a fairly homogenous look in many galleries.