The 44-year-old actor stepped out unannounced and presented a clip from his forthcoming film "Sherlock Holmes" to an enthusiastic crowd of more than 6,000 Comic-Con fans Friday.
Downey called Holmes "an intellectual superhero" in the tale set in England in the 1800s. The film is based on the original stories by Arthur Conan Doyle and the era's obsession with the occult.
"We reinvigorated it by changing it less" than other adaptations of the story, Downey said.
Also starring Jude Law as Dr. John Watson and Rachel McAdams as Holmes' love interest, Irene Adler.