My idea was to have Karloff play a retiring horror film actor; the old footage was used as a film-within-a-film he’s making at the end of his  career. So to contrast the illusion of horror in Karloff’s gothic castle with real horror, a deranged Vietnam vet starts sniping at people at a drive-in. This was a few years after the Texas Tower incident and Charles Whitman. Roger [Corman] really liked this idea.
I showed my original script to Sam Fuller and he liked it too. Then in three hours in his office he virtually re-created the script for me of the top of his head as I took notes. Roger said of the revise, “This is the best script I have ever had to produce”.

Peter Bogdanovich, in Roger Corman and Jim Jerome, How I Made a Hundred Movies in Hollywood and Never Lost a Dime, Hachette Books, London 1998

Once Bogdanovich finished working on The Wild Angels, he was able to set about preparing his first film, albeit under Corman’s strict conditions. A 130,000 dollar budget and just 11 days of shooting, with the participation of Boris Karloff who still “owed” Corman two days of filming (luckily, later they would become five) under his contract for The Terror. It was a serious limitation, not least because the limited time with Karloff meant involving another supporting actor. Bogdanovich came up with an original solution to the problem. Dictated by a contingent necessity, the solution ended up being the film’s most successful idea. With his wife, and guided by the advice of a consummate director like Samuel Fuller, he wrote a story based on two parallel narratives that meet once at the beginning and then converge in a single ending… With this unusual and compelling first film, immediately acclaimed by the critics upon its release, he laid the foundations for his future by clearly setting out the themes he would develop in his other films: the personal joy of mise en scène; trust in narrative and action mechanisms (here doubled, and referring to the dualism in his personality, according to the director himself, between “emotionality” and “intellectuality”); the relationship between criticism and film, cinema and life; the exploration of film “genre”; the expressive use of colour; film as an example of directing and as a reflection on cinema.

Vittorio Giacci, Peter Bogdanovich, Il Castoro, Milan 2002