I don’t think they are ever hot in the OT. Plus wouldn’t the hilt get too much to handle after a while, if they were?
The hilt likely wouldn’t conduct the heat, much like when you hold the handle of a frypan.
Of course in the Phantom Menace we see lightsabers melt through a set of strong looking metal doors in the first five minutes of the movie. Compare to Luke cutting the hatch on an AT-AT, which should theoretically be sealed better than “a space door.” *
In a fantasy adventure film, it’s about doing what is in service to the story, and to maintain a certain pacing in the film. In TESB, Luke had to find an opening in the AT-AT, in order to plant his explosive. Taking 5 minutes to cut open a hatch would have been ridiculous, and greatly slowed down the pacing of the scene.
It’s less about what is “realistic”, and more about “what looks cool and will keep the audience interested?”.
The heat has also been used to explain why lightsaber wounds aren’t horribly bloody… (Someone please tell Ponda Baba)
That particular shot (of the arm on the ground) wasn’t filmed by George Lucas - from memory it was done in the US by the second filming unit (I don’t recall the exact details off the top of my head).
It’s relatively easy to adjust that shot though – I think that Adywan’s edit changed the arm. This is something I’ll do for my own edit.
Remarkably, in Return of the Jedi, we see Luke’s saber acting like a baseball bat against Jabba’s thugs. Not sure if it’s hot or not.
During filming of ROTJ in 1982, Lucas had a baby to raise, and stated that he wanted to make more of a child-friendly film, which means the film’s violent shots were reduced or removed, and the film became more cute and happy. Aside from that, if there were limbs being lopped off, and lots of visible wounds on Jabba’s cadre, the film wouldn’t have gotten a PG rating back in 1983.
*Ok I guess it’s not just a space door; the room must be sealed if the Jedi are to be poisoned, but who has poison cannisters in their AC system?
The production team did two things:
#1 Took inspiration from some real-life office building safety systems: fire suppression systems that activate when there are fires in certain rooms (such as a CRAC unit or Server room). This gas is poisonous! These systems lock the doors, and flood the room with fire suppression material.
#2 Did things to push the story forward – as mentioned above: doing things in service to the story. The Neimoidians want to get rid of the Jedi, so they activate their version of an internal fire suppression system, which in the case of this fantasy adventure film, is a type of poisonous gas.