BBC One, BBC Two.

At the risk of making him blush, why this guy?

I'm trying to think of a clever answer about who was unavailable, but...

I can't believe it.

Well, here we are.

It's an atomic test.

The Russians have a bomb.

We're supposed to be years ahead of them, but this...

The truth is I've... all of us in Hollyw⏤every director knows this guy sitting next to me,

who, for the listeners, is Robert Downey Jr., if they're not watching a picture.

He's... he's one of the greats and, you know, of late, I've seen his superhero chops, his incredible charisma...

I am Iron Man.

... in an amazing series of films and... and he's become one of the great movie stars of our time.

And I am Iron Man.

But, for me, I... I really wanted to see him lose himself in a character and... and just show the world,

and I wanted to be involved in that process of showing the world, that he's just one of the great actors.

And now, the race is against the Soviets.

- Not unless we start it. - Robert, they just fired the starting gun.

I mean, I'm sure everyone's saying this, but I forgot it was you.

Sometimes I thought you were Jeremy Irons just for a second.

- That's harsh, man. - To Jeremy.

Wait. Jeremy Irons in what?

I was gonna say, you got to be more specific.

I'll take it.

I want to ask something.

I've not got much time, but the number of cuts in this film, because⏤what was the assembly for this?

- As in how long? - How long?

I don't know whether I should admit this on Radio One, but we don't really do an assembly.

My editor Jen Lame⏤actually, second film I'd done with her⏤she came on to the film after we wrapped because she was busy on "Wakanda Forever".

So, she came on and we started working together with the dailies, which is, is a process I really enjoy.

We go back to the beginning, we watch absolutely everything, and then we start to put it together, and it's a very, very complicated film to put together.

Yeah, isn't it just?

- I mean... - I can only imagine; my mind blows just thinking about it.

And the opening shot is raindrops.

How set in stone was that for you?

Not set in stone at all.

It's not in the script actually, which, for me, is very rare.

It's a sort of symbol and a symbolic representation that started to insert itself in the filming.

I'm a very controlled and controlling filmmaker and I don't often shift something as important as that.

But it was something that just kept pulling us in and pulling us in and we kept repeating in the filming.

Myself and Hoyte van Hoytema, Cillian, you know, just finding this.

And working with Jen in the edit suite relatively late, we realized that... that's exactly the opening.

He's not bad, is he?

I mean, you don't know the half of it, brother.

I... everything that has ever been done at the highest level on any other film I've ever done was all done simultaneously, and then some.

I came in to do ADR, where in a film of this scope, you usually have three days of work.

He had one line that he only⏤and it wasn't even a line that they didn't record correctly.

It was a line that he wanted to add from the previous draft that he remembered because nothing escapes him.

It's bananas! And you have to live it to believe it.

But it's all⏤it's not about control; it's about his dedication to this art form and bringing it in the most powerful format possible.

And I don't want to give anything away, but there's a sensational moment of audio when the test occurs.

No spoilers, no spoilers.

I would love to have been in the room⏤the ADR room, as you said⏤recording those voices.

Extraordinary; you must have felt incredible capturing those⏤

I have no comment to add here other than that.

No, I'll just say that we... we didn't capture them in ADR; we... we used for real recordings.

- See? - Yeah. Yeah.

All right, one last thing.

A friend of mine who works for QI, so, really knows his stuff, a big geek.

I told him the incredible, wonderful fact that the IMAX reel of this film is 11 miles long.

He discovered that it takes three hours to walk 11 miles on average.

So, you can watch "Oppenheimer" at the same time as you can walk it.

I wanted to give that to you.

- Oh, man. - Thank you; that will never leave me.

In IMAX.

Can I tell you, we finally got something back from the BBC.

All this take, take, take with these interviews and, finally, here's the fact for you.

I don't quite know what to do with that fact, but it's interesting.

I'll stick it in your birthday card, "Never forget, three miles long."

I smell meme.

That was a freebie.

-Congratulations. - Thank you.

Thanks for watching.

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You can listen to my radio on movies and TV, podcast, screen time on BBC Sounds.

And you can find these interviews in full on BBC iPlayer by searching "movies with Ali Plumb".