Interviewing Stephen Hawking is a very strange experience for a journalist. Even meeting him is a little daunting. He is of course, a giant in science. So you know that behind those spectacles, is a very sharp brain.
But the Motor Neurone Disease that has afflicted him for 50 years and left him unable to move means that there are more outward signs of that.
I say "hello". He cannot respond. He's just arrived in the lift at his office in the Department of Mathematical Sciences at Cambridge University.
One of the two carers who accompany him all the time leans across his motorised wheelchair and holds Stephen's hand out, offering a handshake for him. I shake the limp hand and say hi again. No response.
When it comes to the interview, we have submitted our questions days before so he can prepare are his answers in advance.
He does this with a gadget in his spectacles which sends out a beam that controls his computer. He builds up sentences word by word, sometimes letter by letter and then send it to the voice synthesiser that has now become his trademark almost.
The robotic voice speaks his answers with a long gap between sentences. My problem is that I can't see his computer screen during the interview and I don’t know when he's finished.
But it's sorted - everything seems to be sorted with Stephen. His carer who can see the screen gives me a nod when he reaches the end of an answer.
So yes, the stereotype of a giant intellect trapped in a twisted body is true. Stephen is very aware that's why he has become such a celebrity - his office is decorated with photographs of him with Barack Obama, Nelson Mandela and the Queen.
But he is above all a scientist. His place in scientific history is guaranteed by his discoveries in cosmology - for example, that Black Holes don't suck in everything - they give OUT thermal radiation - heat.
And whatever question you ask him, his answer is rooted in science. Is there a God? We are only intelligent monkeys on one planet among probably in the Universe that might support life. A Creator is unnecessary to explaining our existence.
Fracking? We will need the gas when the lights start to go out.
Hawking is now 71 - when he was diagnosed with ALS (a type of motor neurone disease) he was given two years to live. That was 50 years ago. He's still confounding the doctors.
He says he knows every day could be his last and he's dedicated to passing on his love of science - not just to his graduate students, but to everyone.
We leave Cambridge feeling that we've been in the prescience of a unique, great man. Uplifted. And happy to have met a hero.
'Hawking' will be the Opening Night Gala of the 33rd Cambridge Film Festival on 19th September, which will be attended by Professor Hawking, and then in cinemas across the UK from the 20th.
It will be available to own on DVD, Blu-ray and digital download from Monday September 23.