The guy who played Lex Luthor on Smallville (Michael Rosenbum?) had a podcast and he talked a little bit about her last year.
I don’t remember all the details, but apparently she was a very normal person for most of the show’s production. But, late into the series, she got involved this cult and her spiral into weirdness happened very quickly.
I believe the VAST majority of actors are already teetering on mental illness. It doesn’t take much - like a cult, for instance - to push them over the cliff.