I know that it's been said that Hanz Holzer had minimal involvement with the seance sequence. Was he ever on the set? I don't think that I've ever seen any pictures of him there. Nor do I recall any stories from the cast.
An interesting question. I reread the stuff that Darren Gross has written about the film and strangely enough, especially considering it's one of his favorite sequences in the longer version of the film, he hasn't really written about its production. And I checked out a couple of the publicity articles that mention how Holzer was involved as a technical adviser. Famous Monsters of Filmland, for example, says, "The seance scene promises to be one of the most nearly authentic ever seen on the screen as it was supervised by a world-famous authority on psychic phenomena." And that press release about the seance scene (that I shared most recently in Reply #1015) says, "Expert Hanz Holser, who has authored many books on psychic phenomena, was called in to supervise, and every detail is exactly as it would be if an authentic medium were running the show." Given they say "supervised," that would seem to imply he was present during the shooting - but it is interesting that for one, none of the actors involved in the sequence seem to have talked about him, and for two, a photo of him on set doesn't seem to have surfaced. But who knows?