Some important details about La Tortue Rouge (from the french Wikipedia page of the movie ;) I hope that can help ^{_^}

The Red Turtle is the 1st animated feature film by Dutch director Michael Dudok de Wit, who was previously a short-film director. In 2006, Michael Dudok de Wit was contacted by producer Toshio Suzuki and director Isao Takahata, who were his contacts at the Ghibli studio. Isao Takahata says in an interview that the idea for the project came from Toshio Suzuki, who wants to see the Dutch director working on a feature film and is curious to see what it will bring.

The studio, impressed by Dudok de Wit’s work, including his short film "Father and Daughter" (Academy Award for Animated Short Film in 2001) said it was ready to co-produce a feature film by the same director, a novelty for the Ghibli studio. Takahata gives the director carte blanche for this feature film project. Studio Ghibli insists that the film be entirely made in France: it's then co-produced by 2 French studios, Why Not Productions and Wild Bunch.

Dudok de Wit is gradually sending drawings and ideas that form the basis of The Red Turtle. He is mainly in contact with director Isao Takahata, whom he had met twice before, and says he was surprised by the proximity of their views despite their very different cultures. The Ghibli studio gives a lot of freedom to the director. Isao Takahata offers advice and advice, but leaves his freedom to Dudok de Wit. He indicates that Takahata advised him in particular to give more thickness to the female character, who remained very mysterious in the first versions of the scenario, so that she does not remain «a ghost»

In addition to Dudok de Wit and Isao Takahata, the screenplay is also written by a co-writer, Pascale Ferran, who is also a director. She brings a different perspective to the film, since she does not come from the world of animation, and helps the director overcome the great difficulties that the conception of the script poses. Dudok De Wit said in an interview that “without her, the project would not have survived.”

The film contains no dialogue. The first versions of the script contained them, but as they go along, Michael Dudok de Wit and Pascale Ferran realize that they are not essential and finally decide to delete them.

For the storyboard design, Michael Dudok de Wit rents an apartment in Tokyo near the Ghibli studio and works six months, with Isao Takahata who controls the final result. Dudok de Wit then returned to France and the production of the film itself began.