He was the hugely successful British actor who made surly professor Severus Snape one of the most watchable characters in the Harry Potter films and garnered widespread critical acclaim.

But behind the scenes, Bafta-winner Alan Rickman grew ‘frustrated’ with his Hogwarts role and was racked by doubts over his impact in the blockbuster films.

The actor, who died in 2016 aged 69 from pancreatic cancer, reveals his misgivings in his archive of personal papers covering more than 40 years on stage and screen.

The collection, with a price tag of £950,000, is being sold by the TV actor Neil Pearson, who has another career as a seller of rare books. The archive reveals how Rickman’s concerns about the role emerged in 2002, soon after he finished the second film in the series, Harry Potter And The Chamber Of Secrets.

Daniel Radcliffe wrote a postcard to co-star Alan Rickman wishing him a Merry Christmas

Nicole Kidman wrote a letter to Alan Rickman saying a stage performance was 'inspiring'

A card from Harry Potter author JK Rowling to Alan Rickman who played Severus Snape

In a postcard, David Heyman, the producer who cast Rickman, told him: ‘Thank you for making HP2 a success. I know, at times, you are frustrated but please know that you are an integral part of the films. And you are brilliant.’

RADA-trained Rickman, who rose to fame with the Royal Shakespeare Company, was still having doubts about the character while working on 2009’s Harry Potter And The Half-Blood Prince, the sixth film. On the set, he queried the character’s ‘narrative and character development’ by writing down his thoughts under the heading Inside Snape’s Head.

He wrote: ‘It’s as if [the director] David Yates has decided that this is not important in the scheme of things i.e. teen audience appeal.’

The star kept all eight of his Harry Potter scripts and they are in the sale. The archive reveals how Rickman built close relationships with author J. K. Rowling and his young co-star Daniel Radcliffe.

Rickman with Emma Watson, Rupert Grint and Daniel Radcliffe in the third Harry Potter film

Shortly after the 2011 premiere of Harry Potter And The Deathly Hallows Part 2, the last film in the series, Rowling wrote: ‘Had to send a line about what you wrote in the souvenir programme for Hallows II. Made me very tearful. Thank you for doing justice to my most complex character.’

His archive also includes touching letters from co-stars including Kate Winslet, who starred with him in 2014’s A Little Chaos, which he also directed. She wrote: ‘Dearest Al with thanks for your guidance. I loved every second.’

Rickman was also an acclaimed stage actor, and another of his archived letters was sent by Hollywood star Nicole Kidman who watched him from the audience in a London production of Private Lives and later wrote: ‘Such a great performance – inspiring.’

Rickman starred in a host of other hit films, including Die Hard, Truly Madly Deeply, Robin Hood Prince Of Thieves and Love Actually.