Who played Natasha Romanoff in 'The Avengers'?

Who played Steve Rogers in 'Captain America: The First Avenger'? In which film is Steve Rogers a character? Who played Loki in 'Thor'? In which film is Loki a character? Who played Bruce Banner in 'The Avengers'? Who played Clint Barton in 'The Avengers'? In which film is Clint Barton a character? Who played Nick Fury in 'The Avengers'?