Ah, the knight in shining armor, or the future warrior in his battle suit. If you write, chances are you're going to write about armor of some kind. The most obvious setting is fantasy or science fiction, but any historical piece may have armor, and the military still uses body armor to this day. Those helmets aren't for show.
Often, armor is treated as a set of clothing, something you throw on and go out to meet the world. People are portrayed as sleeping in their armor, moving about without noticing it, never maintaining or caring about it, and so on. Now, if you're going to write a very romanticized, fantastical version of a story, that can happen - its just assumed to be part of the setting. But if you want a believable, plausible setting, you're going to have to do things a bit differently.
The first thing to understand is that armor is neither comfortable, nor easy to wear. Even the most modern, top of the line, best designed, and high tech armor still is heavy and awkward. Armor is a trade off between weight/discomfort and safety/protection. It will keep you safer at a cost, but the cost is always there. Think of it in terms of a coat. Without the coat, you're colder. With the coat, you're bulkier, heavier, and sometimes get too hot.
In Vietnam, a lot of soldiers died because they refused to wear that helmet. Why? Because when the temperature is over 100 Fahrenheit and the humidity is in the 90% range, everything is miserable, but the hat was especially so. So was that flack jacket body armor they had. And there were a lot of casualties because of it.
The further you back in time, the more pronounced this becomes. As recently as the early 20th century, fabrics we take for granted and wear regularly did not exist. Baseball players wore wool outfits in the blazing heat of summer of 1939 not because they liked it, but because that was the best they could get. So if you're going to be historical, then you're going to have to work with the restrictions and materials available at the time.
Where did this come from? Well a lot of it probably can be laid at the feet of Mark Twain's A Connecticut Yankee in King Arthur's Court which asserted satirically the suits were so heavy you had to use a crane to seat them. This really took hold but even 100 pounds isn't all that heavy worn as a suit.
So why the squire? Well unlike video games, you can't actually carry 12 weapons so the squire handles that (usually on another horse or a cart), and it takes a while to put all that plate on so its good to have help. Plus, the squire was a manservant, to deal with problems like cleaning up messes, making beds, packing and unpacking, and so on. Most squires were also knights-in-training, trying to learn the job so they could be one when they grew up.
SEEING IS BELIEVING
And weight is just one issue. Consider a moment; this helmet is a very high tech, very protective knight's helm from the medieval period:
Massive protection, very difficult to get any sort of hit in on something like that. But what can you see from inside there? Something like this:
Now, you can get used to that, but its still going to be very limiting and claustrophobic. The more visibility you get, the more face is exposed, and especially eyes. All helmets restrict vision to some degree, even if its a slight distortion from the see-through portion or a bit above where the top overhangs. That's just a price you pay to survive. Of course if you are very skilled and experienced you can get used to it and learn to work with the limitations, but always it will be limited.
So its not unreasonable that someone could miss something like a bad guy to his side or moving around him because of the helmet they're wearing.
Now, I've been emphasizing the weight and restriction of armor, but I want to make sure that I am not misunderstood. Some people take this way too far, making knights in full plate unable to stand back up, get on a horse, etc. The fact is, that plate armor was designed to be as flexible and useful as possible. A full suit of plate armor wears around 50 pounds, whereas jousting armor would weigh over 100 pounds. but that is distributed around your body in a way designed to make it sit well and not feel as heavy.
Part of the reason people get the idea that heavy plate was nearly immobile was because most of the surviving armor we see and people used to study was for jousting and the tournament. Those suits were so heavy and so huge that they literally could be difficult to stand up and move in. Those suits were ridiculously thick and heavy, and did not come designed to allow mobility and long use.
The reason is simple: if you are jousting, you don't need to move around much. The entire point is to sit on your horse and take a hit, so the armor was ridiculously heavy and thick, and not very mobile. But nobody wears that in real combat, because its designed for one purpose.
Think of that somewhat like a dragster vs a street sports car. Both are fast, both are designed to drive, but the dragster is extremely fast and designed to do only one thing: go straight as quickly as possible. Your Lamborghini is designed to go around corners, stop quickly, drive smoothly, and so on. Its not as fast because it has a different purpose; its meant to go on roads and travel long distances. Claiming all cars are like the dragster is to not understand its purpose.
Real combat plate armor was well articulated and while heavy was not as heavy as the jousting stuff. Here's some video of people wearing period-accurate real armor using metal available at the time:
As you can see, people are significantly more mobile than often they are portrayed. It weighs you down, its heavy and restrictive, but you're still able to act and move well in it. And a trained warrior would build up muscles and skills that let them function even more effectively. It will be tiring, necessarily, but endurance can be built up.
The truth is, in a good suit of armor, you can run, dodge, dance, duck, crawl, and leap over things. I've seen footage of a man leaping over a fence, then vaulting onto the back of their horse from behind. He was tired, but it could be done. As you can see in the video, people are cartwheeling, wrestling, etc. It was like a (heavy) skin, designed to allow flexibility and mobility.
Now, another thing you might have noticed is the sound. People wearing and fighting in armor sound like an earthquake in a kitchen. All that metal is tied down as best you can, but it still rattles about and is absolutely noisy, even when walking around. Even chain mail makes a fair amount of noise as you move, as does leather (squeak squeak). So you're not likely to sneak up on anyone in this stuff.
FIGHTING IN ARMOR
As a practical concern, its worth considering how you fought someone in armor. Obviously the point of a suit of armor is to allow someone to stand and fight enemies with melee weapons such as swords or axes and suffer little or no damage. Ideally, a suit of armor will make you invulnerable to the usually poorly trained and lightly-armored (if any armor) peasants and footmen.
Ideally. However, armor did not make you invulnerable, and a good hard hit will transmit energy through the metal to your body beneath, so you're going to end up bruised and banged about. Put even a modern motorcycle helmet on your head and let someone knock on it and feel how loud that is up against your ears and enclosed. While people would wear a layer of padding beneath their helmet, its not nearly as good as modern foam and other materials, and frankly it would ring your bell to get hit in the head, even if you didn't suffer any significant physical damage.
Armor is designed to not present any flesh to the enemy. Unfortunately, armor that is completely enclosed is also immobile, so some level of flexibility had to be built in, which means... gaps.
Some parts were almost completely enclosed, such a helmet where mobility was not an issue (visibility, as I noted above, is another matter). This is an example (show piece for Henry II) which demonstrates how closely it closes up. You can see the plates overlap and close together so that there are no gaps, but more importantly look closely at how they overlap.
Medieval designers learned that if you had the overlaps facing forward, a blade would slide along the armor and into the gap, and hence into the face of the occupant. this way, an attacker even if they come from behind will not hit the target's face even if they slip something between the gaps.
But not all armor can be made that way, and some of it even was only on one side, especially on the legs. A knight was typically mounted; they are cavalry. Mounted warriors need not wear armor where they are touching the horse, and indeed would prefer not to, since its hard on the horse and uncomfortable on the rider. So the legs would have heavy armor on one side and cloth or light armor on the back.
Even full suits had gaps at the joints, in order to allow mobility and flexibility. The armor could not grind or ram up against its self or you wouldn't move around well (and do somersaults, etc as shown above). And that means that fighting a knight was about either wearing them down until they are too tired to protect themselves... or hitting them where their armor was weakest.
Now, its not like you are naked under a suit of armor. A full suit of real plate was a series of layers, from a sort of longjohns suit of underwear to a layer of padded armor to protect against impact, to sometimes even a layer of chainmail at least at joints, and then the plates on top. That means no part was completely exposed, but it didn't get the full benefit of those heavy metal plates.
In really well-fitted armor, the gaps were minimal: between the legs (usually covered by a skirt of articulated plates), under the arms, etc. A good suit of armor was tailored literally for the buyer, built around their body to fit smoothly. and neatly. Areas of particular vulnerability (knees, for example) would be given extra protection with small plates. Straps and joints were inside or riveted and bolted so they were not easily targeted.
A warrior would aim for joints, overlaps, and gaps if possible because not only are you doing minimal damage to someone in a suit like this, but you're damaging your weapon. Those swords are very nice looking, but they have the same kind of edge as your kitchen knife. I wouldn't recommend doing so, but swing that chef's knife against the side of a soup can a few times as hard as you can and see how well it cuts after that. The damage is visible.
And against any edges, such as plates of armor, there will be chipping and even shattering of hard steel, which destroys your sword. Will the guy in the armor feel it? Absolutely, but he'll feel impact, not slicing or stabbing damage, and your weapon is being destroyed.
I'm not going to say much about period armor and keeping it accurate. If you are writing a historical novel, you're going to want to make sure you're more careful than, say, the TV show Vikings, about what kind of armor and equipment the combatants carry so you don't mix time periods, but generally speaking fantasy is pretty loose about this sort of thing.
One small tip to consider: there is no historical support for the "studded leather" armor. You cannot find a single example of it in actual armor. The terms seems to have come from images that showed armor with rivets or pieces of metal apparently on them, but what that was probably showing was brigandine: a layer of metal plates or rings sewn or riveted between leather. All that shows on the outside are the connections, which looks like studding, but that would provide no actual armor benefit on its own other than decoration.
So, if you write your armor, try to keep it reasonable and plausible, using armor the way it was used in the past - its not just another shirt, but its not a horrendous weight you need a crane to get up while wearing.