What was full the extent of Val Kilmer's difficult behavior on set?
I know that Joel Schumacher made no bones about how hard it was directing him. He said that Val was verbally abusive/hostile towards the crew for no apparent reason. When Schumacher told Val to knock it off, Val stopped talking to him for at least two weeks.
https://the-boneyard.com/threads/actors-that-should-have-been-bigger-stars.123120/page-2
This is partially the reason why Val Kilmer wasn't brought back for Batman & Robin. The official reason is that there was a scheduling conflict with The Saint. Although even without that issue, Joel Schumacher wouldn't have asked him to return anyway.
I've also heard rumors in the past, that Val Kilmer had an affair with Drew Barrymore, and that was the impetus for his divorce from Joanne Whalley.
https://moviechat.org/tt0112462/Batman-Forever/5ec21ae9c0a9d8349066242b/Did-Drew-Barrymore-really-help-break-up-Val-Kilmers-marriage