Robert Downey Jr. thinks he'd be in jail if he wasn't an actor.

The 'Oppenheimer' star - who was arrested numerous times on drug-related offences in the 1990s and endured two stints behind bars - joked his career has kept him on the straight and narrow and his life would have been very different without it.

Asked during the Hollywood Reporter's Actors Roundtable what he would do if he wasn't an actor, he laughed: "If I wasn’t an actor, I would be doing hard time.

"A trumped up felony, possession again..."

His admission prompted 'American Fiction' actor Jeffrey Wright to say: "If I wasn’t doing this, I’d be a lawyer — maybe, I don’t know, maybe a criminal lawyer. I’d be Robert’s lawyer."

Robert quipped: "It’s a growth industry."

The 58-year-old actor also admitted he once walked out of an audition for a commercial.

He recalled: "I went in for a Dr Pepper commercial. They came out in the hallway and they said, 'OK, you’re the Pied Piper and everybody wants to be like you.'

"And I just stood up and walked out of there.

"So I didn't have a chance to be bad."

Robert began his career in independent movies before taking on the role of Tony Stark/ Iron Man in 'Iron Man' but he never hesitated about joining the Marvel Cinematic Universe (MCU) because he had so much faith in director Jon Favreau as their outlooks were so similar.

Asked if he had paused before signing on for a studio superhero movie, he said: "No, because anyone who knows Jon Favreau — I remember seeing 'Swingers', and that monologue he has, and I was like, 'And he wrote this? Who is this guy?'

"Also, he went to Bronx Science, and he was doing the Improv in Chicago, and we’re both from Queens? We were meant to do this thing.

"Also, there was no real certainty that this was even going to take off. 'Iron Man' was a second-tier hero.

"They [Marvel] let the lunatics run the asylum for a little while, so it was completely an indie approach to a genre movie to begin with."