From a long Scottish newspaper article about the book, film and filming – timed to coincide with the end of the Glasgow Film Festival and the looming release of The Eagle film in the UK:

(Kevin) Macdonald is using the novel as a framework for a film brimming with authenticity and yet which unfolds as something of a modern boys’ own adventure, a tale of bravery and friendship. “Kevin and I have been calling it Brokeback Eagle,” laughs the film’s leading man, Channing Tatum, when I join him in his centurion’s quarters in the furthest quarter of the fort.

“There has been a lot of joking about that, because there is this male friendship that binds the story together,” he continues. “Obviously, it’s not really a homoerotic element, but it is inherent in a story about two people who come from different backgrounds, who go through an awful lot together and hate each other at times, but end up with respect and friendship for one another. You could say it is a buddy movie in the classic tradition.”

The two buddies (in the film The Eagle based on the Rosemary Sutcliff book The Eagle of the Ninth) are Marcus Flavius Aquila (Tatum), a young centurion who arrives in Britain in 127 AD, and a Briton, Esca (Jamie Bell), who becomes both retainer and friend.
Source: ‘We’ve been calling it Brokeback Eagle’ – Herald Scotland