The unbelievably uncomfortable "chat" - in which Guru-Murthy tried to ask the actor about his past drug use - prompted a less-than-impressed Downey Jr to walk out of the room.
However, Guru-Murthy doesn't believe he overstepped a mark by asking about the actor's "dark periods", and has defended his antagonistic line of questioning.
The Channel 4 News presenter attempted to steer the conversation away from the usual promotional stuff, and ask questions about Downey Jr's past drug addiction and his relationship with his father, filmmaker Robert Downey Sr.
He said: "The reason I’m asking you about your past is you’ve talked in other interviews about your relationship with your father and the role in all of that of the dark periods you went through, taking drugs and drinking and all of that. I just wondered whether you think you’re free of all of that or whether that’s something…"
But he didn't get to finish his sentence as Downey Jr asked: "What are we doing?" and then walked out of the interview.
Writing in the Guardian, Guru-Murthy explains: "I prepare for Hollywood actor interviews the same way as any other, by reading and watching what people have said before."
The broadcaster goes on to argue that, as Downey Jr had spoken about his drug use in the past, it was fair game to ask him about it while he was promoting the latest Avengers film.
"There were two things from past interviews that seemed interesting for a Channel 4 News audience: Downey had told the New York Times he couldn’t go from a $2,000-a-night hotel suite to prison and come out a liberal, and he’d suggested to Vanity Fair that drug abuse had an inherited element," Guru-Murthy says.
"None of it should have come as a surprise, but I nonetheless offered him two opportunities to say: 'I’d rather not talk about this stuff'. He could have engaged more with the earlier questions and I’d have never had time for the ones he didn’t like. He could have played a dead bat with the serious stuff and the whole thing might have been dropped from the running order as too dull."
So, future advice for Robert Downey Jr: be more boring.
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