The key quote in Diane Keaton's memoir does not come from Diane Keaton. Nor from one of her unbeatable back catalogue of exes (Woody Allen, Warren Beatty, Al Pacino and the unconfirmed but fairly inevitable Jack Nicholson). Nor even from Dorothy Hall, Keaton's late mother, whose disconsolate diaries are spliced with her daughter's story  homespun sourdough added to the jammy showbiz. It comes from Cher.

"There is only value to having the look you have when you are young," Keaton quotes the Mermaids star as saying, "and no value to the look you have when you are older." It is an astonishingly boring thought, hard to get through without a nap. What's significant is that Keaton feels moved to waste space with such generic soundbites rather than risk her own.

It's not a one-off. Almost everyone else's insight gets priority treatment. On religion, Keaton opts not to give us her own view, nor to call into service one of the number of thinkers who have devoted some time to the subject. Nope. "Grammy Hall summed it up best: 'Anybody tells you they've died and gone to heaven is a dirty liar.'"

Another example. While publicising The First Wives Club, Keaton and co-stars are asked what's better about being 50 than 20. "Goldie [Hawn] plunged in with: being a great mom; learning how to grow up and love yourself for who you are; coping with the discomfort of fame; loving a man but not holding on too much ..." This continues for some considerable time, before Keaton wraps things up with: "What could Bette [Midler] and I add? Goldie had said it all."

This isn't simple self-effacement  it's professional and personal deference taken to dismaying new heights. What makes Then Again fascinating is picking apart why she insists on doing this. After all, Keaton won global adoration, as well as an Oscar, for "playing an affable version of myself". Wouldn't that provide a life-time boost in self-confidence?

But what Keaton homes in on is that Annie Hall was "famously inarticulate"  and it's true nobody wants a book full of la-di-dahs. Plus, of course, she was given such exposure by someone (Allen) with a vested interest. Keaton goes further: "Without a great man writing and directing for me, I was a mediocre movie star at best."

Her abashment would be cheering if it weren't so compulsive. Then Again grinds no axes, squeezes no sour grapes, just licks the boots of those who've given her a leg-up; even if they've later knocked her down. Pacino was overwhelmed by her desire for him. While she was just "artistic", he was an artist, a spirit it would have been a crime to shackle  "Sometimes I swear Al must have been raised by wolves."

Beatty was "smart, lawyer-smart ... a mind-blowing dream of drop-dead gorgeous", who "opened every door with his bullshit detector fully charged". Allen is a genius with a "great body" whose "insights into my character were dead on and  duh!  hilarious" and who is now "borderline repulsed by the grotesque nature of my affection". Even Nicholson was so generous and great at snogging when they shot Something's Gotta Give that it's this iffy 2003 romcom which Keaton picks as her favourite film.

Then Again came into being as an opportunity for self-sacrifice, a means of giving Keaton's mother, who died in 2008, the fame that she, if not craved, then certainly felt to be missing. Dorothy Hall's diaries, by turns poetic and fuming, were her only comfort through a protracted middle age.

Keaton credits her dreams coming true to Dorothy, and says she "continues to be the most  influential person in my life". Might it be because for one night, when Diane was nine, her mother found herself in the limelight, crowned Mrs Los Angeles 1955 for her domestic nous? "Something had happened, a kind of betrayal. Mom had abandoned me, but, even worse, much worse, I secretly wished it would have been me up on that stage, not her."

There's something complicated going on here  admissions that feel more the stuff of a psychiatric suite than a celeb memoir: admissions of hours spent trying to widen her eyes in the mirror, and of attempts to seek adoration for fear of intimacy; an equation of her mother's diary-writing with her own struggles with bulimia in the early 70s. (The paragraph listing her vast daily diet is extraordinary.)

Plus, of course, Keaton's path has been forged partly in opposition to her mother's. At 50, Keaton adopted a baby, Duke; another, Dexter, was adopted soon after. "At 54 Dorothy was put out to pasture with 32 more years of living staring her in the face. At 65 there is no pasture, and I'm not lonely." For a book so concerned with genetic bequeathment, Keaton doesn't really engage with problems that might be caused by the fact that your children don't share your DNA. It's a pity. "Everyone is sort of adopted, in that eventually we're all abandoned in one way or another," she writes. "What constitutes a family? Hard to say." But what you want is for her at least to attempt an answer.

Then Again is a charming book  a testimony to kindheartedness. But it's also at times a troublingly raw reaction to recent bereavement. This feels keenest when she addresses not us but Dorothy. "Oh, Mom, Mom, you were such a game girl in so many ways  Am I misrepresenting you? I can only hope you would have forgiven me for revealing your demons  You would have wanted me to share it, right?"

One wonders how Keaton will feel about Then Again in years to come, whether she might consider it was too rapid a reassessment. For, in such strenuous cheerleading for others, she reveals more about herself than may have been intended.