Based on the oldest known written story in a language purporting to be English.
According to Ray Winstone (Beowulf), he and his fellow cast spent several days filming in blue skintight suit, "showing up all your lumps and bumps in all the wrong places. Which can be hard when you're standing in front of Angelina Jolie (Grendel's Mother), who looks stunning in hers."
Crispin Glover and producer and director Robert Zemeckis had previously been on bad terms stemming from the decision to use Glover's likeness in Back to the Future Part II (1989) without his permission, which led to a major lawsuit. The two ultimately reconciled, and Glover agreed to play Grendel.
The name "Beowulf" is a kenning of the Anglo-Saxon words for "Bear". A kenning is a phrase that is substituted for the usual name of a person or thing. It is typically composed of two terms, with the first word added to the second in a way that conveys a meaning neither word has alone. Therefore, "Beowulf" comes from "Bee-Wolf", meaning "Bear".
Model Rachel Bernstein (uncredited) was the body double for Angelina Jolie (Grendel's Mother) in the nude scenes.