I know films like Dracula and Frankenstein were taken from books, but did they make it on stage? Did audiences thrill to a splash of fake gore and a bloody knife?
Absolutely!
Horror has always been a big part of theatre - right back to the plays of the ancient Greeks.
Of course, in that era, the actual horrors would occur off-stage, but they definitely existed - like the eponymous Medea, who murders her own children at the end of the play named after her; or Oedipus Rex, in which the title character stabs out his own eyes.
However, things really got gruesome in the later Elizabethan theatrical world, where, for example in Shakespeare’s Titus Andronicus, all the stops are pulled out - it includes rape, torture, mutilation and even cannibalism.
As you mentioned Frankenstein, you will be happy to learn that there were several early stage adaptations of Mary Shelley’s horror classic - the most notable being H.M Milner’s Frankenstein: or, the Man and the Monster, and Richard Peake’s Presumption; or the Fate of Frankenstein, which debuted in 1823!
Shelly herself watched it - and in a letter is quoted as saying “But lo and behold! I find myself famous!”.
Here’s a playbill for the play:
http://shelleysghost.bodleian.ox.ac.uk/?location_id=105#Description
Horror theatre in the truly modern sense of horror found its purest expression in France, not England - with the development of the “Grand Guignol” theatre. Particularly under its second director, the Theatre du Grand-Guignol specialized in shocking horror shows: allegedly, they used the number of people who fainted in the audience as a marketing ploy ...
A curious cross-over between cinematic horror and the Victorian horror theatre in Britain was the career of actor Tod Slaughter. His work was already defiantly old-fashioned when he began touring productions of such plays as Maria Marten: or, the Murder in the Red Barn and Sweeney Todd, The Demon Barber of Fleet Street in the early 20th Century. He then brought his work to the screen. Like the plays, the films didn't find favour with critics of the time and rarely played prestigious venues but they were popular enough, and cheaply enough made, to be profitable for over a decade. The films were period-set and often criticized for their dated melodramatic style so perhaps give us the most vivid flavour of the original Victorian horror theatre, at least in its less elevated form.
It's also worth looking at the 1887 production of Dr. Jekyll and Mr Hyde as performed in the US and UK for many years by American actor Richard Mansfield. His transformation scene impressed West End audiences so much that he was suggested as a candidate for Jack the Ripper. More seriously, this adaptation of the novella has influenced film versions of the story to this day. The fact that it played Broadway and the West End indicates that horror theatre, in a suitably literary form at least, was not the sole province of the more populist, down market, venues.