Weight has always been an issue. Two years ago it all came to a head when I was thrown off a flight by Southwest Airlines for being "too fat". The airline suggested I was too big for one seat and that I needed two. Now, I'm the first to admit I'm fat, but I'm not that fat and never have been. Astonishingly, for three awful days, it became the world's biggest story. But more of that later.

The first time I became body-conscious was when I was nine and went to the water park with my cousin. We were having a blast going down the slide when this dude at the top goes, "Sorry, man, pregnant ladies are not allowed on the slide." The first five people behind me were like, "Heeheehee!" I went and put my shirt on… for the rest of my life.

I was brought up a Catholic. My mum used to say, "Kevin, we all have crosses to bear, and being fat is yours." It shapes your attitude to life, defines your character. Fat made me what I am – as a person and as a film-maker. If I had been a thin kid, girls would have just landed in my lap. But fatties don't get picked first – in sport, as a boyfriend, anything. So we have to work harder and try to give something else. Some dude will put on the right clothes, put his hair back, look picture-perfect and that's his move. He walks into a room and things just come to him. When people like me walk into a room, we have to start working to win people back, because we're not the physical paradigm and girls start looking for somebody else. But if they give you enough time, and you're funny enough or clever enough or interesting enough, that's a different set of plumage you can show off.

I decided to make movies that reflect who I am and who my friends are – films such as Clerks, Chasing Amy and Dogma. I didn't want to tell stories about the pretty people. Other people could do that. Pretty's great to look at, but pretty ultimately loses to interesting. It's weird that I became a film-maker, because film is such a visual medium and I am not a visual guy. I'm best with words. That's why my movies, particularly the early ones, are so static. I'm like, "Who gives a shit about the picture, who gives a shit what it looks like, it's what they're saying." That comes from being fat, too.

So what is my kind of movie? Basically, it's one where guys don't take off their shirts when having sex. Whenever I see anyone make out in a movie, the first thing that happens is the dude takes off his shirt, or the chick does, and that seems so foreign to me. That doesn't look like anything I know in the real world. My wife, Jen, has been trying to get me to take off my shirt for the 14 years we've been together, but I won't. You could probably count on two hands the number of times she has seen me topless. It's all to do with self-perception. Jen thinks I'm crazy. She is thin as a rail and she's hot. If I could be as thin as her, I would never have had a problem in my life – or so I like to think. And still every day at one point or another, she whines about being fat. Oh my God, I think. If you reckon you're fat, I'm going to keep this shirt on until the grave, because I know what's under here.

When I met her, I was at my adult thinnest – 230lb. I don't know what I weigh these days, but if I did I wouldn't tell anybody. I give out everything to the world, but that number I keep for myself, because it has power. The moment you put a number on it, all the small people in the world have it to throw back at you. I don't think I could live if I weighed myself regularly. The moment you get on the scale, the scale becomes your destination. I'm a creative person, I don't want that.

Could I be a thin person? Possibly. I could certainly do more exercise. When I'm at home, I do this mile-long walk near my house, and if you do it once a day, it keeps you in a good place. But I couldn't run. The moment I'd start, I'd think, what must I look like with my flab going like this? What do people see? Does that dude have a camera phone? But being fat in Hollywood, among the beautiful thin people, had never been a problem. You're just Big Guy to everybody. "Hey, Big Guy!" "Yo Big Guy!" Nobody calls you fat any more. It helps if you're always out there calling yourself fat, too. For a long time I would do that and I lived in a wonderful little bubble where I was fat but it didn't matter.

Then the Southwest thing happened and that burst the bubble. It was February 2010 and I was travelling back from Oakland to Burbank. The rule for flying on Southwest is that passengers who are unable to lower both armrests when seated should book an extra seat. Southwest claims this policy is a result of complaints it has received from customers whose comfort has been ruined by the "encroachment of a large seat-mate". I have been travelling all my adult life on planes and have had no problem lowering both armrests. But the Southwest staff chose to disagree. The captain said I was a safety risk and ordered me off the plane. I'd been seated at this point for around 15 seconds and there was no way the pilot could see me. I said, "You've pulled the wrong guy off the plane." They said, "Here's a pamphlet – you can write to the airline." I said, "Don't worry. In about one hour, you guys will come looking for me." At the time I had 1.6 million Twitter followers (now more than two million). I started tweeting.

"Dear @SouthwestAir – I know I'm fat, but was Captain Leysath really justified in throwing me off a flight for which I was already seated?

"Wanna tell me I'm too wide for the sky? Totally cool but fair warning, folks: if you look like me, you may be ejected from Southwest."

On and on it went. Southwest Airlines had embarrassed me. I was going to embarrass them right back. After almost an hour of tweeting, somebody got scared shitless because the manager came to find me at the airport. He sprinted over and breathlessly coughed, "Mr Smith, please stop tweeting!"

He told me I had been handled poorly and he wanted to make it up to me by ensuring I got out of Oakland as soon as possible on the next Southwest flight to Burbank. Mr Manager escorted me on to the flight as soon as the doors opened – which was embarrassing because everyone else in line stared.

Even though I had the whole plane to choose from, I immediately hurled my fat ass into the first available seat. The most important thing was getting my seat belt on without incident. Once safely buckled in, I started tweeting some more.

"Dear @SouthwestAir, I'm on another one of your planes, safely seated & buckled-in again, waiting to be dragged off in front of the normies.

"And, hey? @SouthwestAir? I didn't even need a seat belt extender to buckle up. Somehow, that shit fit over my 'safety concern'-creating gut."

I snapped a picture of myself with my cameraphone and tweeted what became a very popular image accompanying every rendition of this story: me in one seat, puffing out my cheeks to look fatter. I included the message…

"Hey @SouthwestAir! Look how fat I am on your plane! Quick! Throw me off!"

I continued tweeting until takeoff…

"The @SouthwestAir Diet. How it works: you're publicly shamed into a slimmer figure. Crying the weight right off has never been easier!"

It was tough making all those fat jokes at my own expense, but I figured it would be worth it. I thought the story was about how a corporation got caught screwing the little guy again, except this time, the little guy had a lot of mouth and even more Twitter followers. But look how long that sentence is. Why on Earth did I expect the media to understand all that when there was an easier, funnier story staring right at them? And that story was Fat Guy In A Little Chair.

Almost every news outlet spent the next three days stripping away my humanity, as people discussed me more like a concept than an individual. My picture graced the Philadelphia Daily News with the slug, "Blimp landed." I learned first-hand that fat people are the recipients of the last remaining socially acceptable prejudice. Racism and sexism will get you ostracised in more enlightened communities, but you can mock fat people all you want. When Southwest made me a national laughing stock, it was like a second coming of the nine-year-old me at the top of the slide. In that moment, I kind of died.

Those three days were the worst of my life. I know all things are relative – three of the worst days of your life could be when you're imprisoned in a basement wondering if you're ever going to escape. But in the safe world of Kevin Smith, they were the three worst days of my life. I'd wake up, remember what had happened and feel depressed. My wife was looking at the stories online, and every morning I'd ask, "Where is it?" and she'd reply, "Top of the Google page." I'd go, "Entertainment?" And she'd go, "No, news page." In the afternoon, I'd go, "Where is it?" and she'd say, "Still top story." Then at night-time I'd ask again and she'd say, "Still top." There were 5,000 articles on Google, all calling me fat. I asked my publicist what I should do and he said I don't know, dude, I've never seen anything like this.

It was Larry King's final series on television and he asked me to come on the show. I was like, fuck, now I'm going on Larry King, but it's not to talk about my work, but some bullshit that an airline put me through. I'm like Octomom now; I'm a freak show. And I didn't want to be a freak show on their terms, so I didn't do it.

It's only now I realise how irrational I was being. After all, I'd always been fat and got along just fine. But all my life, whatever front I'd put on it, in my head I had always told myself, "Don't be fat, don't be fat, don't be fat." And when they really shone a light on me, it wasn't for the movies, it was for my fatness. Then, after three days, Tiger Woods gave a press conference where he talked about cheating on his wife and nobody cared about me any more. I don't even like golf, but now he's my favourite athlete ever.

Although it died away, a lot of rubbish was said about me and my weight. Newspapers reported that I lost 70lb after the Southwest incident. What actually happened was that I put on a bunch of weight after Southwest, and then lost it all again.

Two years on, it still seems like a nightmare, but I realise in some ways it has changed my life for the better. It was sink or swim, and I decided to swim – on my terms. I thought, I'm not going to fly when I don't have to. I'm going to get a bus, I'm going to bring my friends and we're going to do a podcast tour. Then I thought, let's take all these podcasts and build a radio network. So we did that. I've taken that line of thinking further – why depend on a distributor when we can distribute films? It has taught me that we don't have to rely on corporations. We can do stuff for ourselves.

As for my body image, that's still pretty much the same as it ever was. I know there are chubby-chasers who think I'm attractive (my wife's one of them), but I don't feel like that and never will. Jen says she likes my body, but I don't believe her. If she does like my body the way she says she does, then I question her taste in everything. But I do know she likes what I am, and that counts, too.

• Tough Sh*t: Life Advice From A Fat, Lazy Slob Who Did Good, by Kevin Smith, is published by Titan Books at £18.99. To order a copy for £15.19, including UK mainland p&p, go to theguardian.com/bookshop, or call 0330 333 6846.