Scarlett Johansson and Sam Rockwell sang 'Happy Birthday' to the Ray Gunn composer

Netflix's animated neo-noir "Ray Gunn" had its world premiere at the London Film Festival on Saturday, and director Brad Bird turned the Royal Festival Hall stage into a birthday surprise. The premiere happened to land on composer Michael Giacchino's birthday, so Bird invited the film's stars, Scarlett Johansson and Sam Rockwell, to sing a "sped up and succinct" version of "Happy Birthday," Variety reports. Giacchino marked the moment by taking a selfie of the whole group. The Royal Festival Hall crowd got a film and a party in one night.

Johansson voices Venus Nova, a famous pop star and the love interest to Rockwell's Raymond "Ray" Gunn, a small-time private detective. She said the role was never a hard sell, because Bird was attached. Johansson and Rockwell were both in London for the event, joining Bird and Giacchino onstage.

"I really was so excited to work with Brad, I'm such a massive fan of his. It was a dream of mine to collaborate with him on anything — I just wanted to work with him so badly."

Johansson added that Bird wrote "very complex" characters, and said the process felt different from her past animated work. "I've worked with directors that direct animated movies," she said, "but it was such a different experience working with Brad on this because he's coming at it from also this other artistic perspective."

Bird, for his part, confessed the whole idea began with a musical mix-up. He said the film came "idiotically after misunderstanding a song" — the B-52's sci-fi-inspired 1979 track "Planet Claire."

"When it started I thought it was a new version of Henry Mancini's 'Theme From Peter Gunn.' But then I was like oh, no, what is this? It's Ray Gunn! And then I went, that's cool and it went from there. So it was inspired by music."

When to watch it

"Ray Gunn" will play in select theaters on Dec. 4 before arriving on Netflix on Dec. 18. The London Film Festival continues through Oct. 18.