I just re-watched The Return of the King, and I couldn't help but wonder whether they really built the Beacons of Gondor on mountaintops and lit them in order to get the shots used in the movie, or at least some of them. It would strike me as quite a task to haul the logs up there, whether by vehicle or helicopter, but it also looked quite realistic. Does anyone know if they really built the beacons atop the mountains in real life, or whether they just animated the scene?
Two (2) different beacons were constructed and set alight by the film team. All others were made in some manner with special effects.
The first is the initial one that we see Pippin ignite.
The beacon that begins to enter the shot from the left at the time position embedded within the following video shows the second beacon that the production team built and genuinely set ablaze. They helicoptered in the necessary resources for the beacon itself, as well as the huts for the guards.
All other beacons involved the use of special effects.
Note: text in brackets at end of blockquote is an addition of mine to aid the narrative
The following quote is directly regarding the second (2) beacon.
"The beacon you're looking at here is actually built on the mountains. We helicoptered the beacon in and the little hut that these guys live in and we flew around in the helicopter and shot it being set on fire. But what you see here is real; none of this is a special effect apart from the distant beacon that lights at the very end [of the shot]." — Peter Jackson, Film audio commentary, 1:06:03
According to an article on Polygon for the 15th anniversary of The Return of the King with visual effects supervisor Christian Rivers the suggestion seems to be that only the first beacon was "actually lit".
The article editor writes (unsourced) that:
The pyre that Pippin (Billy Boyd) lights is real, which meant that the stakes of the shoot were high.
Return of the King’s soup-eating Gondorian guard tells his LOTR story - Polygon
It later quotes Rivers as saying:
"They actually lit it, and the idea was that we’d look up, and then quickly put the fire out" ibid.
This seems to suggest not only the surprise from the visual effects supervisor that they used real fire. But also that they wouldn't leave a fire burning for a long time. The article goes on to describe how Rivers in his nervousness flubbed the first take and that the pyre had nearly entirely gone up in flames:
"The thing that actually caught fire, the whole pyre was about to go up."
This seemingly suggests they weren't interested in having large fires go up with people nearby. Further supporting the idea that the latter seven beacons were animated and not really lit.
As noted by bob1 in the comments, the scene was filmed at Mt Gunn which is located in a National Park. New Zealand has specific laws for the setting of fires in National Parks and the crew almost certainly had to acquire permits for their fires and to ensure they did no damage to the surroundings. Lighting that many bonfire-esque pyres would've meant plenty of paperwork and increased risk dramatically.