The 21-year-old has says he was already overwhelmed with emotion to be at the screening of Yardie - and that's before Idris made a surprise proposal to his girlfriend.
"It was very unexpected, such a beautiful moment," Adnan told Newsbeat.
The Luther actor popped the question to girlfriend Sabrina Dhowrie on stage at an east London cinema yesterday.
"Yardie was the first screening I've ever been to," said Adnan, from south London.
"Idris came on stage and thanked everybody for their hard work. He called his now fiance on to the stage and thanked her for being part of the journey with him, and then he proposed."
Yardie is Idris' debut as a director, and the screening at Rio cinema in Dalston was for the cast and crew.
"I was already emotional because I was about to see myself on screen, but that made me even more emotional," said Adnan.
"We all stood up and clapped. I'm grateful I was there to witness it. It was such a beautiful moment. And that set the tone throughout the whole film for me.
"The lights went out and I already had tears in my eyes. Sabrina was so happy."
The film tells how a young Jamaican makes it from the streets of London to the top of the drug-dealing underworld.
Adnan plays Engin, a 'wannabe bad boy', who like him is British-Turkish.
"I've been acting professionally for about a year now. But this was the first time I auditioned for somebody I could relate to, somebody who came from the same place as me.
"I grew up in south London, where acting wasn't really accessible for me."
Adnan says he never planned on becoming an actor, and can't believe he has had this opportunity.
"It's been an amazing experience, being directed by Idris Elba. And because he's also an actor, he knows what help to give us.
"After the screening we all went to a private venue, celebrated the film, and the proposal.
"It's a great film that I'm proud to be part of, and it was nice to go up to Idris and say 'Thank you for this opportunity, for choosing me'."