How does the anime for Excel Saga differ from the manga? More specifically, does the plot in the Excel Saga anime depart significantly from that in the manga?
I've read that some characters only appear in the anime. What are notable differences between the characters who appear in the manga and those who appear in the anime? Are there any other differences? Is the structure of the anime (i.e. one episode spoofs of different genres) different from that of the manga or are there similarities?
I've only read a bit of the manga, but as far as I can tell, the manga was more of a focused satire on the state of Japan's economy and politics in the '90s and early 2000s (a bit more like the later, similar series Nerima Daikon Brothers). It didn't have as much genre parody as the anime did. It also had the character of Elgala, a third ACROSS agent who works alongside Excel and Hyatt.
Sometimes production houses tend to adapt the story line a bit or go in a completely other direction. Why? I don't know exactly, there could be multiple reasons. but fr instance Hellsing made it's TV adaptation, which wasn't all that bad. However the readers of the manga found a lot of differences, and were not happy about the animated version. Later a new version labeled as an OVA (Official Video Animation) was produced, which had it's story based and continued like it went in the manga. A pretty much similar thing happened with Full Metal Alchemist, where the story very deviated from the original manga concept( which was later reproduced as Full Metal Alchemist: Brotherhood).
Reason mostly vary but it's mostly because production houses do stuff on their own and then mutate the storyline.
During the development of the anime, Arakawa allowed the anime staff to work independently from her, and requested having a different ending from the one in the manga. She said that she would not like to repeat the same ending in both media, as well as to make the manga longer to work more in the development of the characters. When watching the ending of the anime, she was amazed about how different the Homunculi creatures were from the manga and enjoyed how the staff speculated about the origins of the villains.
For instance... (source)
