For those who know Peanuts chiefly from the TV specials, movies or greetings cards, it's a funny, sometimes goofy but ultimately heartwarming saga of a kid who can't get a break but whose friends tend to pull together for him in the end. The most famous single Peanuts image features a speech bubble that could have been (but wasn't) invented as a Hallmark slogan: Lucy, the strip's resident psychopath, hugging Snoopy and murmuring 'Happiness is a warm puppy'.

From 1965's A Charlie Brown Christmas to the upcoming The Peanuts Movie, Peanuts on screen has always been warm and kid-friendly. The consistent use of children rather than adults to do the voice acting means that even the harshest line is softened when spoken in the blurry voice of a small child, and averts the shrieking that adult voice actors resort to in the likes of Horrid Henry whenever they want to signify bad temper. Vince Guaraldi's perky soundtrack wraps it all up in a warm blanket of soul jazz. No surprise that The Peanuts Movie has been praised for respecting these traditions.

And that's as it should be; tough material has to be softened if you want to appeal to as many people as possible. Peanuts creator Charles M Schulz wrote A Charlie Brown Christmas, and being no fool he did everything necessary to ensure it would get aired. As such, it originally contained dutiful references to its corporate sponsor, Coca-Cola (cut from subsequent reissues) and the whole thing was conceived as an attempt to portray the 'true meaning of Christmas', something which Schulz, a devout if complex Christian, felt strongly about. The result was genuinely heartwarming, if a bit sweet in the tooth.

Peanuts the comic strip was a rather different animal. The very first Peanuts cartoon was published 65 years ago last month, on 2 Oct 1950, and the final one came out in Feb 2000. It took Schulz a few years to settle on the right drawing style – in that first strip, Charlie Brown's head is a lentil-shaped oblate spheroid, and he didn't yet have the iconic zig-zags on his shirt – but the emotional tone was established from the off. A boy and a girl watch Charlie Brown coming down the street. 'Here comes good ol' Charlie Brown,' says the boy. Charlie Brown walks past them, smiling cheerfully. 'Good ol' Charlie Brown … yes sir!' muses the boy. They watch him go, and once he's gone the boy scowls and mutters 'How I hate him!' Peanuts was always about innocent cruelty; it's a strip in which people are defined by not getting what they want.

Schulz the cartoonist, unlike Schulz the screenwriter, was beholden to no-one, and he used the strip to exercise (and partly exorcise) his own personal demons. The characters in Peanuts are most themselves in how they react to disappointment: Charlie Brown mopes; Snoopy either shrugs it off and starts dancing, or gives into to a fit of 'WAAH!'; Linus quotes the Bible; Lucy uses violence. In a 1970s storyline, Charlie Brown came down with a mystery illness and Linus told Lucy that he wasn't getting better. Her reaction was not to feel sorry for Charlie Brown but to explode with rage: 'I need somebody to HIT!!'

Schulz may have been a Christian, but he never let his faith get in the way of a joke. The characters in Peanuts are notorious for having more knowledge of scripture than is common in eight-year-olds, but when in one brutal Sunday strip from 1970 Lucy offers Charlie Brown the football to kick, he exclaims 'How long, O Lord?'. She notes that he's quoting Isaiah, finishes the quote for him, and muses, as he runs up to the kick, that it could be that Isaiah was 'unwilling to accept the finality of the Lord's judgment.' Charlie Brown kicks, she whips away the ball, he lands on his back, and she leans over him and chillingly says 'How long? All your life, Charlie Brown, all your life …'

Even when Charlie Brown wins, he can't enjoy it. After bowling a perfect game, he comes home triumphantly bearing a trophy with his name engraved on it, only to have it pointed out that they've spelled it 'Charlie Braun'. The Peanuts Movie is about Charlie Brown's pursuit of the Little Red-Haired Girl. In the movie, she's a character. In the strip, we never see her.

The movie will undoubtedly create a new generation of fans, and it's to be hoped that they will find their way to the cartoons, one of the richest and strangest and truest fictions of the 20th century, the great American novel that guys like Bellow and Updike aimed for and missed. A new book, Only What's Necessary: Charles M Schulz and the Art of Peanuts, captures in its title the essence of Schulz's art. The early strips have fussily detailed backgrounds, but Schulz soon learned to leave out as much as possible, making the stage almost bare except for some elemental furniture: a tree, a doghouse, a school desk, a pitcher's mound. Charlie Brown's despairing murmur of 'I can't stand it … I just can't stand it …' is, for me, up there with Samuel Beckett's 'I can't go on, I'll go on' – because of course Charlie Brown will stand it. He will try to kick the football. He will always lose his kite. But he will keep trying.

Life, of course, will keep letting him down. During Charlie Brown's illness, Lucy vowed that if he got better, she'd let him kick the football. He got better, and she held the football, and he ran up, and she didn't pull it away.

Instead, he kicked her arm.

The Peanuts Movie is released in the US on Fri 6 Nov and in the UK on Sat 21 Dec.