Before being cast as Princess Leia in Star Wars, Carrie Fisher thought she knew about fame. Her mother is Debbie Reynolds, star of various MGM musicals, while her father was the crooner Eddie Fisher, who caused a sensation when he left Reynolds and their young children to marry Elizabeth Taylor. But the fame that Fisher knew as a child of celebrity parents was, she later discovered, “associative fame. Byproduct fame. Fame as the salad to some other, slightly more filling main dish.” When she became famous in her own right, she was completely unprepared. “What is happening?” she would ask herself. “How did we get here? Where is here? How long will it last? What is it? Do I deserve it? What does this make me?”

Her sudden leap into the limelight at the age of 19 forms the backbone of The Princess Diarist, which Fisher calls her “sort of memoir”. This isn’t her first “sort of memoir”, but whereas 2008’s Wishful Drinking focused on her mental health (Fisher has bipolar disorder), this one examines her troubles with celebrity and sex.

On the fame front, Fisher, who is now 60, is typically sardonic, adeptly capturing the unexpected madness of Star Wars, which was meant to be a “cool little off-the-radar movie directed by a bearded guy from Modesto. A thing like that wasn’t going to make people want to play with a doll of you, was it?” Had she known how things would end up, she adds, she “definitely would have argued against that insane hair”.

When discussing her relationships, she mostly maintains an air of amusement, though the facts are, if not earthshattering, certainly eyebrow-raising. The big revelation in The Princess Diarist – indeed its very raison d’être, given that it takes up over half the book – is that, during the filming of Star Wars in Elstree, Fisher had an affair with her co-star Harrison Ford, then a married father of two. She kept it under wraps, mainly out of shame.

So why spill the beans now? Partly because of the obvious: Fisher has a book to sell and, if you’re in the business of writing multiple memoirs, such bombshells are better eked out than squandered in a single volume. But also because she recently found her teenage diaries stashed under her floorboards, and, confronted with her 19-year-old self, was taken aback by this young woman whose wisecracking exterior masked an inexperienced, insecure girl who was way out of her depth.

Fisher goes pretty easy on Ford, who first seduced her in the back of a taxi when she was seriously drunk

Fisher goes pretty easy on the then 33-year-old Ford, who first seduced her in the back of a taxi when she was seriously drunk. This was shortly after he’d rescued her from the clutches of some similarly inebriated crew members at George Lucas’s birthday party. He was, she recalls, “just so handsome. No. No. More than that. He looked like he could lead the charge into battle, take the hill, win the duel, be leader of the gluten-free world, all without breaking a sweat.” She also remembers him as emotionally distant, monosyllabic and a bit boring, though this didn’t stop her falling for him. They would have sex at the weekends and act like strangers on set during the week.

That he never properly acknowledged what was happening between them clearly rankled. “If Harrison was unable to see that I had feelings for him (at least five, but sometimes as many as seven) then he wasn’t as smart as I thought he was – as I knew he was. So I loved him and he allowed it. That’s as close a reckoning as I can muster four decades later.”

Fisher also includes her original diary entries, which are rambling, repetitive, overwrought and ultimately not worthy of the generous space that they are given. At one point she remarks that she would be “posthumously embarrassed” were anyone to read them; you can’t help but wish her older self had taken note.

Diaries aside, however, her writing is mostly smart and funny. The pages crackle with self-deprecating one-liners, chatty observations and the singular wisdom that comes with being forever immortalised in the minds of teenage boys in a metal bikini and chained to a slug. Her relationship with the space fantasy that made her famous is clearly a love-hate one – “Star Wars was and is my job. It can’t fire me and I’ll never be able to quit.”

It’s only in the penultimate chapter that Fisher strikes a truly bum note, as she grumbles about the Star Wars conventions where fans and fanatics queue up for autographs, often dressed as their favourite characters, each paying $70 for the privilege. She once said that she wouldn’t be caught dead at these nerd-fests but now, she notes, “I’ve been caught alive at those round-ups often enough to wish I was dead.” Her send-ups of the breathless soliloquies delivered by silver pen-wielding acolytes are just mean.

It is one thing for Fisher to come clean about her relationship with Ford. But for a woman who signed away her rights to Leia merchandise and has intermittently found herself strapped for cash, laying into the fans would seem not just ungenerous but biting the hand that feeds.

• The Princess Diarist is published by Bantam. To order a copy for £13.93 (RRP £16.99) go to bookshop.theguardian.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99.