Actually, Shatner wasn't too bad in this civil rights era exploitation flick. Shatner plays a provocateur inciting the flames of hatred as a small Southern town's high school is about to be integrated.
Filmed in part in East Prairie, Missouri, Corman used that town's high school as an important set piece. However, the sheriff ran Corman and his crew out of town, reportedly because they were thought to be Communists. Corman and an assistant had to sneak into town to get one final needed shot of the high school, fleeing after getting the shot and seeing the sheriff's car coming after them.
For a one-note film, The Intruder has garnered a surprising amount of followers over the years. Part of the reason, I suspect is the screenplay and the source novel, both written by the talented Charles Beaumont. Beaumont was a highly regarded fantasist who died all too young (and all too horribly) at the age of 38; although his by-line appeared up to his death, his friends complete many of his stories without credit. Among his many accomplishments, he had the honor of being the first person to have an original story published in Playboy magazine. Next to Rod Serling and Richard Matheson, Beaumont contributed to more episodes (22 in total) for the original The Twilight Zone. He sold his first story in 1951 and by the end of the decade had moved into television and screen writing. Among his films were Queen of Outer Space (written with Ben Hecht), The Premature Burial (written with Ray Russell), Burn, Witch, Burn (written with George Baxt and Richard Matheson), The Masque of the Red Death (written with R. Wright Campbell), and the absolutely amazing (IMHO) 7 Faces of Dr. Lao. The Intruder was the second of Beaumont's only two novels; the first -- Run from the Hunter, written with John Tomelin and published under the joint pseudonym "Keith Grantland as a Gold Medal original paperback -- was the inspiration for the television series The Fugitive.
Featured along with Shatner are Frank Maxwell (By Love Possessed, Madame X, and a gazillion telesion shows), Beverly Lunsford (whose brief fifteen year career included The Crawling Hand and a recurring role on television's National Velvet), screen heavy Leo Gordon (Hondo, Riot in Cell Block 11, Baby Face Nelson), and Robert Emhardt (The Magnificent Yankee, 3:10 to Yuma, Wake Me When It's Over). Charles Beaumont also acted in the film (as Mr. Paton, his only film role) and his writer friends George Clayton Johnson and William F. Nolan also appeared as, respectively, Phil West and Bart Carey. Johnson and Nolan went on to write Logan's Run.
It has been moted, but unconfirmed, that voice actress June Forey (the voice of Rocket J. Squirrel, Natasha Fatale, Dudley Do-Right's Nell Fenwick, Tweety Bird's Granny, Witch Hazel, Cindy Lou Who, Pogo Possum, and so many others) provided the voice of The Old Hotel Clerk.
The lore behind this film and its filming is interesting but should not detract frommthe fact that this is a pretty good B movie.