"I've been terrified ... and I used to say that I was bipolar, manic depressive all the time and I think it was kind of a good thing to write" in this condition, he said.
The Girl in the Spider's Web, which goes on sale in 25 countries on Thursday and in the United States on September 1st, picks up the trail of tattooed computer hacker Lisbeth Salander and journalist Mikael Blomkvist.
"I was not the easiest person to live with because I was thinking about it all the time. I'm scared to death that I won't live up [to Stieg Larsson]," he said, referring to the writer who created the trilogy but died suddenly of a heart attack in 2004 at age 50, before the series gained global fame.
Larsson's three Millennium books, published in 2005-2007, have sold 80 million copies worldwide and have been made into Swedish and Hollywood movies.
Lagercrantz, who gesticulates wildly while speaking and is fond of superlatives, was visibly anxious about how his book will be received.
"This was the passion of my life and now you can judge if I succeeded, but God I did my best," he said.
Dressed in a dark suit, he described the "fever" that overtook him as he wrote, and how he really enjoyed developing the characters.
"I dream of being Mikael Blomqvist. He's a nice guy with great values," he said.
Stieg Larsson's father Erland Larsson, sitting in the front row, looked on, his expression alternating between amused and serious.
"Stieg would probably have been more relaxed and would have been more cautious," he told AFP.
The English-language version of the book. Photo: Fredrik Sandberg/TT
Among them is Eva Gabrielsson, Stieg Larsson's partner of 32 years until his death.
The couple were not married and Larsson left no will, so his estate went to his brother and father. Gabrielsson, 61, lost a bitter battle with them to manage his work.
In an interview with the AFP news agency in March, she called Lagercrantz a "totally idiotic choice" to write a sequel, saying he knew nothing of the milieu Stieg Larsson described in the books.
"They say heroes are supposed to live forever. That's a load of crap, this is about money," Gabrielsson said.
"It's about a publishing house that needs money, (and) a writer who doesn't have anything to write so he copies someone else," she lamented.''
But publishing house Norstedts rejected the claims, saying all publishing houses bring out books to make money.
It said the Swedish journalist, who penned football star Zlatan Ibrahimovic's official biography, was the perfect choice.
He has a "special talent for depicting the world of others," said Norstedts head Eva Gedin, who edited both Larsson's and Lagercrantz's Millennium books.
She insisted the novel "is David Lagercrantz's book, not a copycat."
Erland and Joakim Larsson have pledged that the royalties from the book will go to the antiracist magazine Expo co-founded by Larsson.
The pair are among the few who have already read the new book, which they have lavishly praised.
"I kept it on my nightstand for a week before I opened it. I was a little afraid. But once I started, it was impossible to stop," Joakim Larsson told AFP.
The writing of the book has been shrouded in secrecy to prevent leaks about the plot. Only a few people have read it so far.
Media interviews with Lagercrantz ahead of Thursday's launch have been subjected to rigorous confidentiality agreements
, angering some outlets, including a Danish daily which has boycotted Lagercrantz because its literary critic was not allowed to read the book before interviewing him.
Norstedts has only revealed that the characters uncover "world shattering information on US intelligence services."
Despite all the precautions, one newspaper kiosk at Stockholm's central station put the book up for sale on Wednesday, a day early, before being ordered by Norstedts to remove it.
A total of 2.7 million copies have been printed, including 500,000 in the US.