Orson Welles was a mammoth talent that never really flourished under the Hollywood structure of his era. He was forced to whore himself out for commercials, he spent decades hawking food and drink, both of which he consumed copious amounts of. Maybe that's why he made such a trusted spokesperson.
It wasn't uncommon for the great director to show up for a session high on spirits and drunk on his own ego. Like this session from the golden age of radio where Welles tried to improve the script he'd been asked to read for a frozen peas commercial. He was notoriously prickly, impossibly eriudite when he was irritated, insulting the sponsor of his program and lambasting the guys in the booth.
Thirty years later, Orson was off-the-charts drunk taping this spot for Paul Masson Wine ("We will serve no wine until its time"). Did they resort to a sound-alike for the finished spot?
That was nothing compared to Colonel Harland Sanders, founder of the Kentucky Fried Chicken chain. He sold the company in 1964 but still fronted their commercials until he died in 1980.
In the seventies he would wander inebriated into a KFC in his neighborhood causing a scene, dressing down the employees because they weren't making the chicken correctly. And there was little they could do when the Colonel showed up for his commercial tapings a bit 'extra crispy'.