"She's a doctor, she's intelligent and a very sensitive and human character," Seydoux said.
In behind-the-scenes footage shot at night, Seydoux is shown running up steps next to the river Thames in London, opposite the Houses of Parliament.
Director Sam Mendes also explained why he had cast the two actresses as Bond girls.
"In Spectre, the two women who Bond hooks up with - both have great mystery, they both have depths and for that you need fantastic actresses," she said.
"And I just thought: Monica Bellucci... She's an incredibly seductive presence in life and in the movie."
He added that Seydoux's character, Swann, "needed to be soulful, feisty and complicated, and I think it's a pivotal relationship. So it probably couldn't be a total newcomer because it needed someone with a certain amount of life experience, maturity".
The 30-year-old Seydoux was the right choice for the part because "she's the whole package", Mendes continued.
In the 24th James Bond adventure, a cryptic message from the past sends 007 on a rogue mission to Mexico City and then to Rome, where he meets Sciarra, the widow of an infamous criminal.
He starts to uncover he existence of a sinister organisation, Spectre, and infiltrates a secret meeting.
Back in London, he enlists Moneypenny (Naomie Harris) and Q (Ben Whishaw) to help him seek out Swann, the daughter of his old nemesis Mr White (Jesper Christensen), who may hold the clue to untangling the web of Spectre.
Spectre will be released in the UK on November 12.
- PAA
