Now and then: Ceratosaurus

Top right: Ceratosaurus was described in 1884 and therefore illustrated in the same way as all "carnosaurs" at this time: semi-erect posture, opulent belly, dragging tail. Ceratosaurus had a narrow crest on its snout, for some reasons described as a horn by the scientists, resulting in artists not looking at the actual fossil illustrating the dinosaur with a conical horn instead of a crest. This morph remained standard until the 1980s. 
Bottom right: the 1980s saw the shift towards more energetic, slim and active theropods having a very lanky body with well-defined muscles. Some muscle groups were simply overlooked (f.e. the caudofemoralis or pterygoideus ventralis), others virtually nonexistent (such as the shoulder muscles; when you can follow the line of the scapulcoracoid exactly in the living animal, you can say it has no shoulder muscles). Also the skull carried very little soft tissue, all fenestrae visible. You can also see the edges of the pelvis and the neck looks like having no place for the trachea and oesophagus. The teeth stuck out of the jaws in a crocodile-like manner and the hands were pronated. 
Left: This began to change in the 2010s. Modern illustrations usually appreciate all the muscle groups, the dinosaurs finally get heaver (in a natural way) and less lightweight and hyperenergetic. Also, it seems to be consensus now that non-avian theropods could not pronate their hands in the "bunny pose" manner. Also, many artists today consider it more likely that the jaws and teeth were covered in lips, although it is still a controversial issue. 

References for the latter two types are Scott Hartman's skeletal drawings, for the old Ceratosaur I had a look at old paintings and drawings from various sources.