[1] Born in Milan to a theatre administrator, Stagni began his acting career at a very young age.

He studied hard to become an actor, mime and circus performer, coached by such figures as Pëtr Šarov, Lee Strasberg, Jacques Lecoq, Giancarlo Cobelli and the Circo Orfei.

Through the years, Stagni's performances onstage were directed by Aldo Trionfo, Franco Enriquez, Orazio Costa, Vittorio Gassman, Gigi Proietti, Giancarlo Sbragia, and Giorgio Albertazzi.

[1] As a voice actor, Stagni performed in a few Italian animated films and dubbed many Disney characters in Italian, such as Mickey Mouse in the 1970s, Br'er Rabbit in the 2nd redubbing of Song of the South, and Donald Duck in the 1988 animated movie Who Framed Roger Rabbit.

He is also Rick Moranis' official Italian voice and has dubbed Alvaro Vitali and Wallace Shawn in many of their roles, also providing the voices of Warwick Davis' Filius Flitwick in the Italian version of the Harry Potter film franchise and multiple characters, including Mr. Crocker, in the animated series Fairly OddParents, of which he also directed the dubbing process.