The film is what led me to read the book. The film is quite violent and has more than its needed share of bloody scenes, but I liked the friendship between the leads and in the credits I saw that it was based off of a book. Obviously, I had to go read it and it was interesting to see the similarities and the changes between the twain.
The beginning is essentially the same. Saturnalia games. They play up his wound far more in the book. Esca becomes friendly much quicker in the book - "I am the Centurion's hound." I found it more realistic in the film that he was more hostile and less willing to just accept the situation, much less embrace it. Honourable enough to be a loyal slave but too proud to ever happily serve, yet that is what it seemed like in the book before they ever became friends. Though Marcus was definitely willing to put all that aside between them from the first. Marcus has a much longer recovery in the book, along with Cub and the taming of the wolf-cub and Cottia, the neighbor girl. I can understand why it was all cut out - though the wolf incident would have been fun. The subterfuge of being an eye-doctor made more sense, as well as the book version of how the Eagle's legion were destroyed. Esca's tribe had no part in it, so there is no big argument between the boys. The search is different given the healer cover. At the point of setting out for the wall both Esca and Marcus in the books are very close and Marcus had made Esca a free man so that he could choose whether to come or not. Their time with the Seal tribe is mostly the same except Marcus is still very much in charge and Esca was not, Marcus being the healer and all. I rather liked Marcus getting a taste of a slave's life in the film and not knowing what Esca was doing. The showdown/escape scene was vastly different. They were smart enough to hide the Eagle and have Esca double back for it but he dropped Marcus's recognizable brooch so the Seals came after them. There was no killing of the kid. There was no huge fight in the ravine. Instead they were chased and chased. Guern helps them, but not the other members of the Legion living locally as in film. A few Seals catch up, but that is all they had to fight - the rest they just had to avoid/escape.
The ending is different. Esca is given Roman citizenship, Marcus is given land and a reward, is married to Cottia, stays in Britain. There were good moments in both with the boys and how each character grows. Too long, boring in parts with no action just time passing very slowly,and even downright sappy in the book and too gory and violent and lacking in time for some serious character development in the film. So, both had their strengths.