There are certainly inaccuracies, particularly in older films where less care was taken over such matters.

But the main thing about modern films is they give a very particular image of a Roman Legionary that isn't necessarily entirely wrong but kind of suggests a greater degree of uniformity between individual soldiers and across space and time that didn't exist so much.

When i say legionary what do you think of? Probably a very rectangular shield, locrica segmentum armour (that's the armour made of stips of metal) and a red tunic. All of these things are entirely plausible but not necessarily uniformly the case.

Shield shapes vary a fair bit between very rectangular in their profile, if curved back and rectangular. That design was probably most common in the peak of the imperial period, the 1st and 2nd century AD. But in much of the republican era it was more of an oval, over time it gets a little smaller and more rectangular as we move towards the imperial period and then later it transitions again so that ovals are becoming more common again towards the 3rd century.

For the armour again that locrica segmentum was primarily a peak imperial period design and even in that peak it was very much mixed in with mail armour. It would have been much less common both before and after that period and probably was still less common than mail even during it peak of use.

Now for red tunics, there is some archeologically evidence to back up the use of red tunics. And it makes sense, red was associated with Mars and war and madder was a common and relatively inexpensive red dye in the Mediterranean area. But firstly there has also been archeologically evidence for the use of simply undyed material. Madder can be used to produce a variety of red colours but generally its probably easier to get out of it various more orange, brick or fiery shades that a really bright scarlet you often see if in films. Given the colour of mars the planet i have also sometimes wondered what shade of red it was exactly that mars the deity was associated with. Plus you have to consider that these dyes would have lost some vibrancy over time with washing and the sun.

Higher ranking officers might have worn a brighter red, Caesar is sometimes mentioned as wearing red to distinguish him from his men. There's also the fact that as i said madder is a Mediterranean plant. Rome's extensive trade networks meant that while you'd probably see red tunics in some numbers everywhere you'd probably get a lot less away from the centre of the Roman world, up on the Danube, Rhine and British frontiers perhaps you'd see less, particularly if you were stationed away from centres of the region. And there might well have been a smattering of other colours, some might have been able to afford to properly bleach their tunics white, some might have worn a blue, some maybe a mustard yellow.

Its also worth mentioning late imperial soldiers seem to rarely get any time in the sun in films and they'd have some slightly different equipment like a longer spatha sword, spears

Yes.

One example is that a lot of Romans in many historical inspired films of some kind or another wear leather or similar bands around their wrists, which isn't what they seem to have done in reality. You can see mosaics of real soldiers and politicians and they are basically never seen to be wearing them in reality.

Another is thinking of everyone in the Roman empire as acting like a citizen in Rome. Especially given that these days we might expect a nationstate to have basically the same language, culture, religion, and food, like in Denmark, and for everyone to be citizens by virtue of being born in the country or being born to citizens living in the country, which is basically everyone who is born in most countries these days, you can forget that this isn't how citizenship worked back then. Some emperors decreed common citizenship grants at periods, but for most of it's history, the people of many provinces were seen to be citizens of that other place under their own kings or polities that just happened to be loyal to Rome, providing some tax revenue to fund an army and if it was agriculturally rich like Egypt or North Africa, send in food shipments. Not even Italians were given common citizenship until the Social War, hundreds of years later.

If you are thinking about a Roman, something that comes to your mind will probably be tied up roughly in the 1st century AD, with the height of the emperors, the military looking like they do with the round rectangular Scutum, the pila, the lorica segmentata, and so on. The Roman Republic might be in the picture but most people don't understand that Rome wasn't a republic like today with an elected legislature up for election every few years and same with a single head of state with a cabinet. The laws were only able to be passed by one of the assemblies, imperium was wielded by magistrates and conferred solely by assemblies, judgement of penalties including executions were done by the people either by jury or by popular assembly, and the Senate, while important, was ultimately consultative.

If people think of inequality in the Roman Republic and especially the Roman empire, people might think of patricians vs plebians. But if they are talking after about 200 BCE, the differences were largely irrelevant to how Rome worked politically. The patricians had plenty of privileges like 6 voting blocks dedicated to maybe a couple thousand patricians in the comitia centuriae, but most among the equitae and some in the first class in the centuriae were richer than the patricians and had much more power in elections. Most senators would ultimately come from them, as would eventually many magistrates.

And hardly anyone ever thinks of the Romans as having ever been a true kingdom, but it was for as much time as America has been a country under it's current constitution. It lasted a long time. But it was vital to understanding the origin of much of Roman culture, urban development, even the Pomerium, and how it grew into a city state able to be the dominant power in the area about 50 km around it.

You probably also think of the Roman military generally as being a highly professional force of volunteer Romans, signing a contract much like today (although for longer periods). For about half of ancient Rome's history, it was based on a citizen militia, of which only about a third of it was even in the maniple system rather than the hoplite system borrowed from the Greeks. For about half of the late Roman history, it was very often foreign and provincial volunteers with few ties to Rome, often you would have a hard time looking at one and realizing that they are not medieval soldiers without a significant background and exposure to the idea that Rome ever operated that way.

People also fail to understand that any culture even in basically ideal circumstances of national unity (like English from 1660 to today) will have substantial changes. Why would the Latin of Romulus or even Julius Caesar sound that similar to the Latin of Romulus Augustus or Diocletian? We get annoyed by sound changes in English from Shakespeare's time, let alone how hard it is to understand Chaucer, 600 years ago.

We also might tend to overemphasize the city of Rome in our imagination during the late imperial age too, Ravenna and Mediolanum were both capitals and more populated, and there is of course Constantinople (called at the time Nova Roma). Some emperors barely knew Rome, many perhaps only going to the city to get confirmed by the senate as emperor and leaving on campaigns for years, often rising through the ranks from the provinces as commanders in armies who ultimately won out in a succession struggle which may or may not have been caused by them assassinating the old emperor.

And this is before I even touch on the continuation of the Roman Empire until 1453.

The Roman Civilization is a very diverse topic, covering 2200 years of history. Could you imagine a single state lasting from almost Hannibal himself pillaging outside Rome until now? What would remain the same over that time? What would be different?