It very much depends on the species. For something well studied like a T. rex, we have a very complete picture of it's general shape, how it hunted and even how it grew (Adults were incredibly stocky but sub-adults were built for speed - in a loose sense, a bit like a cheetah that morphs into a giant Hyena as it grows!) but how social it was, the skin texture, colour and feather coverage is still speculative. For animals like sinosauropterx, anchiornis and psittacosaurus we have complete pictures of their colours and patterning. Some species are often fossilised in groups or mass burials, which is a strong indicator they were social or partially social animals.

Something like spinosaurus is a good example of something we don't have a huge amount of fossils for (well, we actually had no remaining fossils until the 2014 specimen) and you can see just how much it changed as new fossils were discovered and studied. Fossils of large sauropods are usually only a handful of bones so so we're a long way off the species level resolution that we can reconstruct smaller dinosaurs with. Which is arguably why most sauropod species are reconstructed very very similarly. Many fossils are only known from teeth, which are probably more likely to see the biggest changes as new discoveries are made. Especially for some fossil mammals. And then you have some things like Dunkleosteus which only has a few known relatives, and limited fossils - so we have a good picture of how the head would look! But not much else. And finally we have animals like the Tully Monster, which we didn't really know if it was related to us, insects or mollusks until recently due to how basal (and then derived in its own way) it is, and how there aren't really any known modern analogues.

Generally speaking, we have a pretty solid understanding of most fossil groups as a whole, but lesser known species are ones that will be reconstructed as more generic. The general proportions and posture of many groups as a whole will probably not change much, but we will likely see much more variation between individuals as science progresses (although some groups are inherently more diverse than others) - Deinocheirus and spinosaurus are good examples of this.