Producer Dino De Laurentiis initially insisted on an Eighties pop soundtrack for Conan the Barbarian, but director John Milius insisted on hiring his then-unknown school buddy Basil Poledouris instead. Basil proceeded to write one of the most famous classical music soundtracks ever made.
If Oliver Stone had his way, the film was going to be a Continuity Reboot set in the far future, with Conan battling mutants in lieu of or in addition to ape-men. Milius insisted on keeping the setting in the Hyborian age.
A sequel written by original director John Milius titled King Conan: Crown of Iron was written in 2001 and was intended to be a worthy follow-up to the original film (it would have disregarded Conan the Destroyer entirely). The project was moving forward but put on indefinite hiatus when Arnold was elected Governor of California. Eventually the project was canceled.
Milius originally wanted Arnold Schwarzenegger to be the narrator. But executives at Universal had problem with Arnold as the narrator, due to his accent and instead Mako was the narrator.
The novelization, based on an early version of the script, has quite a few. Conan has a fight scene with the dead king that he takes the sword from which rises up and tries to take it back. Thulsa Doom turns out to be a the last of a race of snake men, with the human form just as magically attained as the snake. His guards are another race entirely with face-obscuring helmets to hide the fact, and all but the top level of his fortress is their ancient home which he took over when he bound them to him by magic.
Charles Bronson, William Smith and Sylvester Stallone were all considered for the titular role. A pity, if it had been Bronson or Smith we might have actually ended up with the King Conan movie everyone wants from the start. William Smith did appear in the film as Conan's father.