Beowulf has some simply remarkable animation sequences, especially those showing the title character. I need to study up to see how they transition between the real people and the CGI computerized animations, but in the best scenes it is difficult to tell the actor from the animation. In general though the film appears as animation, which makes it a bit harder to suspend your disbelief. The quality is high enough however that one wonders how long it will be before we can’t tell real characters from animated ones.
The poem Beowulf is one of the oldest surviving stories in the English language. Unfortunately the film takes too many liberties with the actual story, though I suppose Zemeckis could argue that in some storytelling traditions it’s normal to embellish and change things with each telling.
He appears to have embellished to bring more nude Angelina Jolie scenes into the film since in the real Beowulf poem Grendel’s mother is killed rather than … bedded.
Here’s a great summary of Beowulf, the epic poem.