The development team originally had been working on a new game engine from scratch for The Return of the King. With progress stalling about a year into development, the team turned to the engine used for EA Sports' Tiger Woods PGA Tour series, which was similarly built to craft expansive areas with large structures located throughout the world, and used it as a foundation for the engine built for The Return of the King.

EA had originally planned to add Gollum as a playable character. This ultimately did not come to fruition for the PC version and almost every console version of the game, but he was added to the GBA version of the game.

One leftover detail indicating his planned playability is an in-game results screen template with Gollum's face faded in the background, which is located in the Cheat Codes menu of the game. Every playable character in the game has a screen like this, with Gollum being the only non-playable character to have one.

At the end of the level, "Pelennor Fields", there is a cutscene depicting the battle between the Witch King of Angmar and Eowyn where the armor and weapon of the Witch King does not match his film counterpart. His weapon being a large mace rather than a flail and his helmet having a different design entirely. This was an early alternate design that went unused in the Return of the King film. Because this game was being produced alongside the film the developers presumably had no time to change it to his final film design before launch.

The PlayStation 2 version of the game had exclusive online functionality and there were alleged plans by EA to add online leaderboards and host tournaments after the game's launch.

After each level is cleared, the player will reach a results screen that tallies up every kill they got in the level. In the PC version of the game, the player is able to speed through this screen by mashing the swift attack button. This option is not available in any of the console versions.

In the level "Helms Deep", there is a section where the player must climb a ladder to reach the top of a wall, and once atop it they must shoot and kill a certain number of crossbowmen to clear this section and activate a cutscene to get to the next area. As the crossbowmen spawn, they will occasionally push each other out of bounds rendering them unkillable. When this happens, the player will be forced to wait until the game activates the cutscene rather than having it activate upon killing the designated number of enemies.

There is a character-specific skip that can only be performed by Faramir in the level "Shelobs Lair". Normally, defeating Shelob would take 3-4 cycles, but Faramir can defeat her in two cycles if the player hits her with the correct sequence of attacks:

• First cycle: three swift attacks in a row
• Second cycle: shoot an arrow at Shelob to make her jump down, activate Faramir's special ability (which helps perfect mode activate earlier), then hit Shelob with the 4 hit combo, "Swift Justice", making sure that the last hit is done while in perfect mode.

When executed properly, the remaining cycles will be skipped and the end level cutscene will activate, but it will be from an overhead camera angle with the character model standing in the middle of the scene.