Gatiss, who plays Mycroft opposite Benedict Cumberbatch's Sherlock, said fans would learn more about the pair's family background in the new series, which starts on New Year's Day.
He said: "We've had this idea for a long time that Sherlock and Mycroft are like Niles and Frasier Crane and have very ordinary parents who are just lovely people and actually Sherlock is more likely the product of a loving home than a broken one."
Martin Freeman, who plays the detective's sidekick, Dr Watson, said he had nothing to do with his real-life partner, Amanda Abbington, winning the role of his on-screen wife Mary.
He joked: "It wasn't a John and Yoko thing when I said I want my missus in."
Viewers will see the pair marry in the second episode of the new series, which is based on Arthur Conan Doyle's novel The Sign Of Four.
Borgen star Lars Mikkelsen appears in the third episode. based on a short story called The Adventure Of Charles Augustus Milverton.
The show's co-creator, Steven Moffat, said the Danish actor plays "the only character or villain that Sherlock genuinely hates".