Other than that no because all other actors must be British. See related questions for more information.
All Harry Potter cast members had to be British or Irish. The only exceptions were characters from other countries and Zoe Wanamaker who moved to England when she was young.
Some of the extras were also American, so long as they didn't have any lines.
Zoë Wanamaker, who played Madam Hooch, was born American, but raised British. Verne Troyer, who played a goblin in the first film, was American, but his voice was dubbed over by British actor Warwick Davis. American director Chris Columbus cast his daughter Eleanor in the non-speaking role of Susan Bones. That's pretty much it.
There is no janitor in Harry Potter at any time.
No, owls cannot talk. Even in Harry Potter.
Celia Imrie does not appear in any of the Harry Potter films
No, Alan Cumming has never appeared in a Harry Potter movie.
Christopher Lee has never been in any of the Harry Potter movies.
No Harry Potter doesn't have any sibling's.
Rupert Grint had swine flu.
Harry Potter does not have any brothers or sisters.
After going through the filmography of the entire acting cast of The Golden Compass, I did not come across any actors or actresses that have been credited with any of the Harry Potter films to date.
There is no janitor in Harry Potter at any time.
No, only the Basilisk in Harry Potter is in the second book/movie Harry Potter and the Chamber of Secrets.
Harry Potter will appear in the Harry Potter books. If he was to appear in any other published novel it would violate copyright.
No, nor was she in any of the Harry Potter movies.
No, owls cannot talk. Even in Harry Potter.
No, Toby Hemingway is not in any of the Harry Potter films.
There won't be any more Harry Potter books.
Robert Pattinson, who played Cedric Diggory in Harry Potter and the Goblet of Fire will play Edward Cullen in Twilight.