Kirby died Monday in a Los Angeles hospital from complications of recently diagnosed leukemia, said family spokesman Bill Harrison.
A native New Yorker with a distinctively high, husky voice, Kirby played the by-the-rules lieutenant who lacked a sense of humor in "Good Morning, Vietnam" (1987), Billy Crystal's good friend in both "When Harry Met Sally ... " (1989) and "City Slickers" (1991) and Marlon Brando's nephew in "The Freshman" (1990).
In a stage, screen and television career that spanned the last 35 years, Kirby also played Young Clemenza in "The Godfather: Part II" and appeared in such films as "The Basketball Diaries," "Sleepers" and "Donnie Brasco."
"I think he was an incredibly thoughtful actor," said Barry Levinson, who directed Kirby in "Tin Men," "Good Morning, Vietnam," "Sleepers" and a 1995 episode of NBC-TV's "Homicide: Life on the Street."
"He really thought through what he was going to do so he could begin to work up a character, and he had great specificity to the little things he'd bring to the character he was playing," Levinson told The Times on Wednesday.
In the 1987 comedy-drama "Tin Men," in which Kirby played a salesman named Mouse, "he ended up putting wax or something behind his ears to make them extend out a little more, and he played the character as very fastidious," Levinson said.
In "Sleepers," Levinson said, Kirby showed up with the top of his head shaved bald.
"He was always thinking about how to embellish a character, sometimes in a very subtle way, but to bring something else to the table," Levinson added.
Rob Reiner, who directed Kirby in "When Harry Met Sally ... " and the 1984 comedy "This Is Spinal Tap," in which Kirby played a limo driver, remembered his longtime friend as "one of the most generous persons I have ever met in my life."
"I think that sensitivity he had toward other people is what he was able to use in his work," Reiner told The Times on Wednesday.
"He was also able to blend incredible honesty with humor" in his roles, said Reiner, who first met Kirby in the 1970s. "That's a rare combination you find in actors -- that they become so truthful and real and yet are able to inject humor into it."
In the early 1980s, Kirby played on a softball team with Reiner, Crystal and Christopher Guest. Kirby's role as Crystal's best friend in "When Harry Met Sally ... ," Reiner said, was a "natural extension in what was happening in our real lives. We all hung out together and were friends, so the character he played was close to how he and Billy really were."
In a brief statement Wednesday, Crystal said the death of Kirby was "a terrible loss."
Kirby made his Broadway debut in 1991, replacing Kevin Spacey as Uncle Louie the gangster in Neil Simon's Pulitzer Prize-winning play "Lost in Yonkers."
In 1997, he starred as writer Alan Zweibel in Zweibel's affectionate play about his friendship with comedian Gilda Radner in the off-Broadway production "Bunny Bunny."
"I've been very lucky," Kirby told the New York Daily News in 1990. "And in this business, you have to have a certain amount of timing and luck because I know, right now as we're sitting here talking, there's a guy driving a cab in New York City who can put me away."
The son of actor Bruce Kirby, he was born Bruno Giovanni Quidaciolu in New York City on April 28, 1949, and grew up in the city's notorious Hell's Kitchen section. He moved to California in the late '60s.
Among his early film credits are the 1971 drama "The Young Graduates" and the 1973 drama "The Harrad Experiment." He also played Richard Castellano's son in the short-lived 1972 TV situation comedy "The Super."
More recently, he appeared on the HBO cable TV series "Entourage."
In addition to his father, Kirby is survived by his wife, Lynn Sellers; stepmother, Roz; brother, John; and stepbrother, Brad Sullivan.
A private funeral service will be held in Los Angeles.
The family suggests donations be made in Kirby's name to the