Laura Linney joins Sherlock movie 'A Slight Trick of the Mind'

When director Bill Condon asked Laura Linney to star in his new film A Slight Trick of the Mind — starring Ian McKellen as an aging Sherlock Holmes — he hadn’t a clue that he was tapping into Linney’s childhood fantasies.

Linney, 50, who will play Holmes’ housekeeper Mrs. Munro, is no casual devotee of Arthur Conan Doyle’s series of novels about the acerbic detective. Rather, she is a devoted, senior thesis-writing, sweatshirt-wearing Sherlock geek.

“I was obsessed with Sherlock Holmes as a young kid,” says Linney, who will begin shooting the film in London and Sussex, England, in July — her first job since she became a mother four months ago. “You know how some people are into Dungeons & Dragons? I was into Sherlock Holmes. I loved the atmosphere of the stories. I loved the intrigue, his personality. Bill had no idea.”

It’s surprising that he didn’t: Linney and Condon have been pals since 2003, when they shot Kinsey, which scored the actress her second Oscar nomination. Since then, they have collaborated two more times, most recently on The Fifth Estate. (That film co-starred Benedict Cumberbatch, who, yes, plays Holmes on the PBS series Sherlock.)

A Slight Trick of the Mind, based on the 2005 novel by Mitch Cullin and a screenplay from Jeffrey Hatcher, centers on the later years in the fictional Holmes’ life, when he retired to Sussex to keep bees, as Doyle’s stories suggested. Mrs. Munro serves as something of a caretaker, albeit one with a young son Holmes takes a parental interest in.

The story focuses on Holmes grappling with old age and dementia, while remaining obsessed with one unsolved case. It will toggle between decades, depicting Holmes both at age 91 and 63, when he’s working on his final case. “It’s a really great mystery about who Sherlock Holmes is, but it’s also a lovely, delicate movie about what happens as you get older,” says Condon.

The film will also mark a reunion of McKellen and Condon, who worked together on 1998’s Gods and Monsters and have been looking to find another project together since.

“Nothing ever came close, and then I read this and it feels like one icon playing another,” adds Condon, who is anxiously anticipating pairing two of his favorite collaborators on camera for the first time.

“I’m looking forward to the combined talent, skills, and smarts,” says Condon of Linney and McKellen. “Both of them are incredibly detail-oriented and do an amazing amount of work before they get to set, and then they dive off the board and become their characters.”