Karen Black, an actress whose role as the acid-tripping prostitute in the movie “Easy Rider” launched her career as one of the emblematic tramps, vamps, kooks and down-and-outers of 1970s cinema, notably in “Five Easy Pieces,” “Nashville” and “The Great Gatsby,” died Aug. 8 at a hospital in Los Angeles. She was 74.

The cause was ampullary cancer, her husband, Stephen Eckelberry, told the Associated Press. Ms. Black was diagnosed with the rare disease in 2010 and this year raised tens of thousands of dollars for experimental treatment through a crowdsourcing Web site.

Ms. Black, who appeared in nearly 200 film and television roles, projected an unconventional allure. She had close-set eyes that could appear crossed from certain camera angles and possessed an intense sexual charisma that gave her an alarmingly unpredictable screen persona. She nearly and almost single-handedly brought an X rating to one early film role.

Jack Nicholson once called her “the most lucid actress I’ve ever worked with. You tell her where it’s at and she grabs it.”

She brought surprising depth of empathy and vulnerability to a range of not-very-bright characters. In the early 1970s, Ms. Black was one of the busiest leading actresses in Hollywood, propelled by what Time magazine once described as her “freewheeling combination of raunch and winsomeness.

Actress Karen Black as Rayette Dipesto in “Five Easy Pieces,” directed by Bob Rafelson in 1970. (Silver Screen Collection/Getty Images)

“Sometimes she is kittenish. At other times she has an overripe quality that makes her look like the kind of woman who gets her name tattooed on sailors.”

Her background was Chicago bourgeois, but she rebelled by quitting high school to marry for the first of four times. After years of stage work, Ms. Black appeared in “Easy Rider” (1969), the low-budget but ambitious biker movie directed and co-written by Dennis Hopper that exploded commercially.

In the film, Ms. Black and the biker antiheroes (Hopper and Peter Fonda) drop LSD in a New Orleans cemetery to hallucinogenic imagery and pulsating rock music.

The film was meant to evoke the 1960s counterculture and was a riposte to bloated musicals and arch dramas and comedies that failed to connect with American youth at a time of deep social unrest. Of the film’s appeal, Ms. Black once told an interviewer that younger audiences simply “wanted to see people throwing up, smoking grass.”

Nicholson, whose career zoomed after his brief turn as a societal dropout in “Easy Rider,” urged director Bob Rafelson to cast Ms. Black opposite him in “Five Easy Pieces” (1970). As Rayette Dipesto, an open-
hearted, if dim, short-order waitress impregnated by Nicholson’s self-hating wanderer, she earned an Academy Award nomination for best supporting actress.

Over the next few years, Ms. Black was less than discriminating in the roles she accepted and played a variety of idiosyncratic love interests.

They included the randy faculty wife in “Drive, He Said” (1971), which marked Nicholson’s directing debut, and the promiscuous “Monkey” in “Portnoy’s Complaint” (1972), starring Richard Benjamin in a poorly received adaptation of Philip Roth’s novel.

She was the crass and adulterous Myrtle Wilson in the 1974 screen version of F. Scott Fitzgerald’s novel “The Great Gatsby,” which starred Robert Redford and Mia Farrow. The film was lavishly budgeted and picturesquebut almost universally lambasted as a cinematic deadweight.

The next year, she was the sexually teasing starlet Faye Greener in “The Day of the Locust,” based on Nathanael West’s apocalyptic story of 1930s Hollywood. It also bombed.

Ms. Black was mindful of her typecasting as women of easy virtue, and she tried to break away in mainstream disaster fare as the flight attendant who tries to land a plane in “Airport 1975” and in small-budget art films such as the 1974 adaptation of Eugene Ionesco’s absurdist farce “Rhinoceros.”

She received some of the best reviews in “Nashville” (1975), director Robert Altman’s ambitious, Oscar-nominated drama that followed more than 20 major characters and was steeped in the paranoia of the Watergate and Vietnam era. The film, set in the country-music capital, featured Ms. Black as a hard-edged singer. She wrote and recorded several songs for the movie, including “Memphis.”

She starred as a kidnapper in Alfred Hitchcock’s last feature, “Family Plot” (1976), and said she and the director established a playful rapport.

“He found out that I had a good vocabulary, so he would try to catch me not knowing the meaning of a word,” she told the New York Observer. “He would say, for example, ‘Your work today, Ms. Black, has been most perspicacious,’ hoping to catch me up. And I would say, ‘Oh, Mr. Hitchcock, you mean keenly perceptive.’ And he’d get all deflated because he’d lost his own game.”

She won a devoted following for her tour-de-force performances in the 1975 horror-
anthology TV movie “Trilogy of Terror,” based on stories by Richard Matheson. However, her career soon went into decline, and to some degree she blamed prejudice against her membership in the Church of Scientology.

Since the 1980s, she had been featured in dozens of small-
budget films, including “Savage Dawn” (1985), “Dinosaur Valley Girls” (1996) and “House of 1,000 Corpses” (2003), directed by Rob Zombie. She inspired a New York punk band to name itself the Voluptuous Horror of Karen Black.

Karen Blanche Ziegler was born in the Chicago suburb of Park Ridge on July 1, 1939. Her father was a businessman and her mother wrote novels.

As Karen Black, the surname of her first husband, she acted in Chicago theater troupes before moving to New York. In 1962, it was announced she would play the ingenue in the Broadway production of the musical comedy “A Funny Thing Happened on the Way to the Forum,” which became a major commercial and critical success, but she was replaced before the opening.

Ms. Black plowed on in a series of short-lived stage productions, winning critical laurels as a teenager behind a kidnapping plot in Mary Drayton’s drama “The Playroom” (1965).

The performance led to a supporting role in director Francis Ford Coppola’s frisky coming-of-age comedy “You’re a Big Boy Now” (1966) and eventually to “Easy Rider.”

Her marriages to Charles Black, actor Robert Burton and writer L.M. “Kit” Carson ended in divorce.

In 1987, she married Eckelberry, a film editor and producer. Besides her husband, survivors include a son from her third marriage; a daughter from her fourth marriage; a daughter from another relationship; a sister, actress Gail Brown; a brother; and several grandchildren.

Ms. Black was known for giving circuitous answers when interviewers asked about her career choices.

“Some people are comfortable with creating, some are comfortable with changing, some with stopping,” she told the St. Louis Post-Dispatch in 1997. “For instance, policemen stop things. They’re into stopping. I’m not into stopping, and I’m not much into changing. Mainly I like to start things. I like creating. I think you’ll find that actors like to become things, to imagine things, to get a laugh.”