"I know, at times, you are frustrated, but please know that you are an integral part of the films. And you are brilliant," Heyman wrote to Rickman.

Another note penned by Rickman, titled "Inside Snape's Head", points at a difficult working relationship with director David Yates on 2009's The Half-Blood Prince.

"It is as if David Y. has decided that this is not important in the scheme of things i.e. teen audience appeal..." the note reads.

Despite his misgivings, Rickman was clearly beloved by his co-workers: in one note writer JK Rowling thanks the actor "for doing justice to my most complex character..."; in another star Daniel Radcliffe thanks Rickman for gifting him a copy of Catcher in the Rye.

Rickman, who was clued in by Rowling on Snape's fate before the final books had even been published, spoke fondly of the character once the film series wrapped.

"I knew with Snape I was working as a double agent, as it turns out, and a very good one at that," he told the Los Angeles Times in 2011.

The archive, which includes almost 40 boxes of Rickman's personal memorabilia, is valued at £950,000 ($1.7 million).