He has reason to believe that Sir Charles Baskerville was murdered, and that Sir Henry is next.
He has a document that talks of the Hound of Baskervilles, which is supposed to the be the thing behind the murders, because it doesn't want anyone living on the Baskerville Hall.
Sherlock Holmes and Sir Henry Baskerville, being the main characters and protagonists, are "round" or well developed.
Sherlock Holmes is trying to solve the mystery of the "Hound of Baskervilles," and keep Sir Henry safe. Mortimer is the most important stock character, because he helps Watson and Sherlock Holmes in solving the mystery.
Sir Henry doesn't believe he is in any danger, but that will change. Selden is the "flat" character because he is an escaped convict, who is thought to be the one behind the murders, but then ends up being murdered by the "Hound.
"The Hound of Baskervilles," the antagonist, is the mystery behind the murder of Sir Charles and causes many difficulties for Sherlock and Watson, in solving the murder. The atmosphere of the flat is overwhelming as viscous smoke takes over the room.Watson describes the room as 'poisonous atmosphere' and it also describes it as 'thick, it is intolerable'.The confined of his flat wouldn't be an unusual thing.I think this environment helps Holmes to come to a conclusion of many of his deductions by enabling him to concentrate his mind and help him pin point key facts in the case.The Hound of Baskervilles The Hound of Baskervilles is a fictional mystery written by Sir Arthur Conan Doyle.The Hound of Baskervilles was first published into book form in nineteen hundred twenty-seven by Liberty Weekly.The solitude of the flat was also important for Holmes thinking.Watson quotes it a 'necessary for my friend in those long hours of intense mental concentration.This kind of atmosphere which Conan Doyle has created makes Victorians feel at home.A home is a place where it is regarded to be safe so I think that is what they would feel about Holmes flat.