Oscar Isaac was surprised by how "self aware" Justin Timberlake was.

The two actors star together in Inside Llewyn Davis, a musical film about the folk scene in '60s New York.

The pair got on well, and Oscar says the popstar-and-actor was full of surprises as a performer.

"[I was surprised by] how self-aware Justin is. And he can be very self-deprecating. He's an incredibly funny, very energetic guide. He's very aware of what's going on, who he is, that persona," Oscar revealed to the UK's GQ magazine.

The film also co-stars Carey Mulligan and John Goodman, and was directed by the Coen Brothers.

American musician-and-producer T-Bone Burnett helped Oscar prepare for the role of Llewyn, and Oscar soon realised why he and the Coen brothers worked so well together.

"He'd [T-Bone] say, 'Play me a song', I'd play a little something of it and then he'd say, 'Do you want to smoke a bit of weed?' He kept saying, 'Play it like you're playing it to yourself.' I would have that stewing in my brain - 'Play it like you're playing it to yourself? What does that mean, man?' That's how he works and it's no wonder he and the Coens get along so well, because it's the same thing.

"It's kind of a crazy phenomenon: both T- Bone and the Coens, their tone is so specific. You can easily tell what is a Coen Brothers or a T Bone Burnett piece of art. And yet it's not an imposed, precise, calibrated, intellectualised, predetermined thing at all. It's a weird thing when the vibe is set for that to just be drawn out of people," Oscar explained.