In the first role he has signed up for since “Iron Man,” Robert Downey Jr., left, will forsake his high-tech suit for a woolen cloak and cap. Mr. Downey has agreed to play the lead role in the film “Sherlock Holmes,” which will be directed by Guy Ritchie for Warner Brothers, Variety reported. The film is scheduled to begin shooting in October, with a screenplay by Anthony Peckham based on the detective stories of Arthur Conan Doyle and on a comic book, also called “Sherlock Holmes.” Most of the movie will be filmed in London. In the role, which Variety said would plumb Holmes’s brawn as well as his brains, Mr. Downey will have some competition. Columbia Pictures is also preparing a movie based on Holmes, this one a comedy with Sacha Baron Cohen as the title character and Will Ferrell as his sidekick, Dr. Watson.