I know pretty much everyone else who has worked as a math consultant on a Hollywood movie. I worked more hours than everyone else combined. Fine with me, you couldn't tear me away.

Ron Howard hired Apollo 15 astronaut Dave Scott to consult on Apollo 13. He finds reality more interesting than anyone's imagination, he wanted to get textures right, verisimilitude. I was hired at first with a short list of specific goals, such as the problem Nash writes on his calculus blackboard. I know too many mathematicians who claim to have solved the Good Will Hunting blackboard while it was onscreen; we wanted everything we did to survive "freeze framing the DVD".

A film is like joining the circus. Once you're in, you can be asked to do anything if there's a way you can help.

After watching art make 200 panels digit by digit for the Pentagon scence, supposed to be random enough to pass as encryption, I wrote a C program overnight to generate similar panels with a simple settings file they could play with. My childhood anti-drug literature included photographs of spider webs under various influences; the storyboard artists used this to draw the "Mad Shack" scenes, as seen in the film. For the writer Akiva Goldsman, I'd make strange phone calls for example to the CIA to discuss terminology.

I was a go-between to strong go players for the game sequences. I proposed a go consultant but they said I'd do. After one rehearsal where no one could act and position stones at once, Ron Howard said to just act, we'll fix it in second unit. We break for lunch, I pronounce "This is why we don't have a go consultant. A go consultant would commit ritual suicide right now." Russell Crowe responds they'll put my phone number in the credits. Then everyone in the room recites verbatim their most scathing reviews ever, to enormous peels of laughter.

Day to day, the props department always wanted more scribbled paper, or a blackboard needed scrawling. The actors were entitled to ask me anything before scenes; I'd often be around hours for a one minute conversation. There's a Time magazine cover with Nash as one of four faces. Held in an actor's hand one can barely make out the backgrounds on Blu Ray, but Russell Crowe wouldn't pose for that still photograph till he'd found me to confirm that the blackboard was my writing.

Facing a writer's strike, and with second unit costing a fraction of first unit, there were body doubles whenever Russell Crowe's face didn't show. I wrote all the math, so I was his hand double for everything. He got the idea of making his fingers longer with acrylic nails, so I had acrylic nails for six months. I loathed them but it certainly established my loyalty. Try having your friends in a bar tell strangers about this; I'd admit I was a Barbra Streisand impersonator and that they'd believe.

I had my nails done at the same shop in New Jersey that Edie Falco used on The Sopranos. The woman who'd promised to do the work was on a cruise in the Caribbean, so she phoned in. She asked me if I was doing anything else for the film besides being a hand double. I said I was also Russell's love double (a story he later enjoyed):

"Will you be needing an extension for that too?"