I've run Return to the Temple of Elemental Evil three times for three very different sets of players, much less their characters. Nothing in the adventure changed -- not a single Fane, not a single NPC. Same thing, from the book. Nothing changed. Didn't matter a whit which characters showed up, the adventure was the same. The details of what happened in play were the same. I mean, one group had a character that said "yeet!" to the naga in the water(?) fane and died a hilarious death that stuck with me. When that player brought in a completely different replacement character, nothing in the adventure book changed.