The Emperor was briefly mentioned in Star Wars: A New Hope. Was he at that point intended to be a Sith (or general Dark Side user, if the term "Sith" didn't exist then)? Or did that only emerge in The Empire Strikes Back, as a kind of early George Lucas retcon that we didn't even feel at the time?
As Wookieepedia's article on Palpatine notes, the original conception of the Emperor was that he was not a Force-user (and thus not a Sith) and that he was weak-willed and controlled by the Imperial bureaucracy. This version of the Emperor was included in the Episode IV novelization. DVK's answer to another question quotes the novelization depicting the Emperor as weak:
Aided and abetted by restless, power-hungry individuals within the government, and the massive organs of commerce, the ambitious Senator Palpatine caused himself to be elected President of the Republic. He promised to reunite the disaffected among the people and to restore the remembered glory of the Republic. Once secure in office he declared himself Emperor, shutting himself away from the populace. Soon he was controlled by the very assistants and boot-lickers he had appointed to high office, and the cries of the people for justice did not reach his ears (George Lucas [Alan Dean Foster], Star Wars: From the Adventures of Luke Skywalker (paperback; New York: Del Rey, 1976), p. 1, ISBN 0-345-26079-1.)
This description of the Emperor hardly sounds like a Sith Lord. It is a quote from the Journal of the Whills, an in-universe historical record; consequently, the reader is intended to interpret the description as coming from an in-universe historian who would presumably know whether the Emperor was a Sith Lord or not.
The novelization was released in December 1976, shortly before the film in 1977, so it's possible the Emperor was re-conceived by the time the film was actually released. Nonetheless, the novelization was credited to George Lucas (it was ghostwritten by Alan Dean Foster) so it's very likely that the Emperor was still intended by Lucas to be weak-willed and not a Force-user by the film's release.
The term "Sith" did exist in Episode IV -- Vader was described as the "Dark Lord of the Sith" in the Episode IV script. In fact, Vader's title suggests that he is the lord of all the Sith and does not have a master, further indicating that the Emperor was probably not intended to be a Sith Lord even at the time of the film's release. This early use of "Dark Lord of the Sith" for Vader despite the fact that Palpatine was his master has resulted in seemingly inconsistent use of the title among Legends authors.
I believe that the novelization of Star Wars mentions in one place, "later corrupt emperors." Senator Palpatine became the first Emperor and later the Emperor in Star Wars: A New Hope would be one of his successors.
So when I read in the novelization of Return of the Jedi and read that the Emperor in the novel was Palpatine I was annoyed that they had changed their original idea and retconned the history to make the first emperor also the current one.