Although he claims never to have become career damagingly typecast, Sachs has these past 20 years been haunted by the ghost of Fawlty Towers's diminutive Spanish waiter with a tenuous grasp of English and an even more tenuous grasp of common sense. People still shout out Manuel's catchphrase - "Que?" - at Sachs in the street.
"It's very nice to have played Manuel, but there have been occasions when I've wondered about it," he muses. "I was doing Tom Stoppard's Jumpers in the West End. I came out of the stage-door after the show and this very nice couple wanted my autograph. `Lovely performance,' they said. `You've just seen the show?', I replied. `Yes, but we mean Manuel'."
Sachs has pretty much got away from the Spanish waiter with his portrayal of Einstein, a 53-year-old man looking back on his life in pieces-to-camera from his dusty Berlin study. Contrary to the image of the scientist as an other-worldly, almost saintly figure, Sachs plays Einstein as a very human character - capable of passion and poetry. "Some people have recently been unfairly critical of scientists," opines Peter Jones, the producer of the documentary. "They're not expected to have a human side. But here we get a picture of someone as human as any great artist or poet - Van Gogh comes to mind. Einstein was someone who dedicated himself to an intellectual creation which bears comparison with TS Eliot or any other intellectual achievement of the 20th century. There was no spirituality lacking in his work - as has been suggested. I hope the documentary will get people excited again about the notion of creativity in science."
Sachs's Einstein certainly displays a wry sense of humour, giving a cheeky eye-rolling grin, for instance, when remembering how he fooled the doctors into excusing him from school. During extensive archive research, Sachs found a twinkle in Einstein which he brings out on screen. "He had this sparky sense of humour and mischievousness about him," the actor reckons. "His eyes dodged about. He was charming and childish - yet he could also be very serious with those hooded eyes. He was a multi-layered character, a man I'd like to have dinner with."
One of the 20th-century's first media celebrities and an artistic inspiration for everyone from pop group Big Audio Dynamite to playwright Terry Johnson, Einstein holds an enduring fascination. "After the Second World War," Jones reflects, "he became an iconic figure. His face, his hair, his presence symbolised hope."
However, the element of the two-part documentary, part of the BBC's celebration of National Science Week, which has caught the eye of the tabloids is the forensic examination of Einstein's stormy private life (affairs, two failed marriages and an illegitimate daughter); they're interested in raunchiness rather than relativity. "I don't mind if some of the publicity goes all-out for the Einstein and sex angle," Jones laughs, "as long as people watch. Science would come first with Einstein, but he needed his sex and his good Schwabian hams."
With his neat, steel-framed glasses, a dapper jacket and a pink shirt with gold cufflinks, Sachs seems far-removed from the legendarily scatty Einstein. "He's everybody's idea of an absent-minded professor, almost like a music-hall joke," Sachs comments. "But he was actually like that. Reputedly Einstein couldn't even tie his own laces. He never combed his hair or washed, because it wasted too much time. To have this dedication, this enormous drive to achieve, demands a certain ruthlessness. Society benefits, but you pay a price for it - or your relatives do."
But the Berlin-born Sachs obviously has a soft spot for his fellow German and hopes his performance will electrify viewers. "Horizon is often above my head," he observes. "My presence might broaden the horizons a bit. As soon as you get showbiz people in, it creates interest. If they could have got Arnold Schwarzenegger to play Einstein, they'd have been even happier. But although he's got the accent, he couldn't fit into the costume."
A two-part `Horizon' on Einstein is on BBC2 at 9.20pm on Sun and 8pm on Mon