A lot of the credit for the original Star Wars being a decent movie must go to Alec Guinness, who plays Obi-Wan Kenobi and brings a lot of gravitas to explaining the Force and the complicated backstory about the Clone Wars. But Guinness didn't think much of Star Wars when he was filming it — he only took the role after they doubled his pay, and he still griped afterwards.
Over at Business Insider, they quote from a couple of Guinness' letters, as cited in Alec Guinness: The Official Biography. (Disclaimer: I couldn't find these letters in the Google Books edition of the book, but the relevant page wasn't included in the preview.) Prior to accepting the role, Guinness wrote to his friend:
I have been offered a movie (20th Cent. Fox) which I may accept, if they come up with proper money. London and N. Africa, starting in mid-March. Science fiction – which gives me pause – but is to be directed by Paul [sic] Lucas who did "American Graffiti, which makes me feel I should. Big part. Fairy-tale rubbish but could be interesting perhaps.
And once he was on set, Guinness was no more impressed, writing to his friend:
... new rubbish dialogue reaches me every other day on wadges of pink paper – and none of it makes my character clear or even bearable. I just think, thankfully, of the lovely bread, which will help me keep going until next April even if Yahoo collapses in a week ... I must off to studio and work with a dwarf (very sweet – and he has to wash in a bidet) and your fellow countrymen Mark Hamill and Tennyson (that can't be right) Ford. Ellison (? – No!) – well, a rangy, languid young man who is probably intelligent and amusing. But Oh, God, God, they make me feel ninety – and treat me as if I was 106. – Oh, Harrison Ford – ever heard of him?
The mention of "Yahoo" refers to a play based on Gulliver's Travels that Guinness starred in around that time. I love the idea of Guinness wandering around the Star Wars set calling Harrison Ford "Tennyson." [Business Insider]