The two Brits discovered they had a similar sense of humour on the New Zealand set of Peter Jackson's epic and producer Barry Osbourne was so impressed with the friends' film ideas he offered to help fund their debut as screenwriters.
The busy pair have reunited for writing sessions whenever their schedules permit and Monaghan tells WENN that one recent creative spurt has given them both the confidence to take their project back to Osbourne.
He says, "Billy just stayed with me for the last few weeks. He and I have been writing a script for a long time, since we spent the best part of nine months in a tree together making Lord of the Rings. We found each other very funny and we wanted to work together.
"The producer of Lord of the Rings, Barry Osbourne said to us, 'You guys should come up with an idea for a script and I'll produce it.' We both love quality comedy, like Life of Brian and Dumb & Dumber and Spinal Tap and Being There and Pink Panther and we're circling around a couple of things right now that we've co-written with some actors.
"Hopefully, in the next six months or so, we'll have something real."
And Monaghan reveals Boyd isn't the only lifelong friend he made while shooting the Lord of the Rings trilogy - he often hangs out with Elijah Wood and his other co-stars.
He adds, "I see Viggo (Mortensen) whenever I can and Orlando (Bloom) and Ian McKellen and Pete Jackson. We've all remained really close friends."