SIX years ago, when the American sitcom Parks and Recreation was in its second series, Chris Pratt brought in a little video camera to film some behind-the-scenes footage.

While he was recording, his phone bleeped with a text message from a pal. "Uh," he improvised, affecting to sound like a spoiled brat actor drained by the world's demands on him, "it's Steven Spielberg talking to me about Jurassic Park 4." Everybody laughed. Why wouldn't they? This was Chris, after all.

Like shoe-shiner Andy Dwyer, his Parks and Rec character, Pratt was a genial slacker, a big-boned, big-hearted, just-passing-through kind of dude, hardly movie-star material at all. He had dropped out of college, lived in a van on Maui, in Hawaii, had more jobs than a labour exchange, and he had been in the business for going on a decade. Everybody loved Chris. But he wasn't going anywhere fast, was he?

Everybody was wrong. Come the end of 2014, Pratt had starred in two of the most successful pictures of the year: The Lego Movie (£307 million worldwide gross) and Guardians of the Galaxy (£507m). Next week he can be seen playing the lead in a new film, Jurassic World, which no-one is calling Jurassic Park 4 lest anyone be reminded of the eminently forgettable 2 and 3. With Colin Trevorrow directing and Spielberg executive producing, Jurassic World is less another sequel than a rebirth. Just as science found a way to bring back the dinosaurs for Richard Attenborough in 1993, so Hollywood in 2015 hopes it is finally about to hatch a worthy successor to one of the most beloved films.

"It was unbelievable," says Pratt when I ask him to recall seeing Jurassic Park for the first time. He was 13. "I went with my mom and dad, and also our family friends Steve and Elaine and their daughter Jessie. It was kind of a date. We were young kids and they wanted to set us up. They wanted us to grow up and be together because they were all best friends, you know. About three minutes in I forgot that anyone else was in the theatre with me. I was just captivated by dinosaurs." He went back the same weekend to see it again.

All this is said in a tearing, wide-eyed rush, as if he really is a teenager again. In many ways Pratt is 35 going on 13. The Lego Movie's theme song, Everything is Awesome!, could be his signature tune. You can see why everybody still loves Chris. His sunniness is on a par with Maui's, his sense of humour sharp and self-deprecating. Before he embarked on the press tour for Jurassic World he issued a "pre-apology" for whatever he might accidentally say that would offend anyone. "I am not in the business of making excuses," he wrote on Facebook. "I am just dumb. Plain and simple." A nice guy, then. A smart one, too. But of course there is more to him than that. Pratt is the stuff of his past and present, as we all are, and those turn out to be a little more complex than first glance suggests.

Jurassic World opens 22 years after the original. The theme park is now up and running properly, those initial teething troubles - a rampaging T-Rex, mass casualties - have been smoothed over. Jurassic World is safe, shiny, but still thrilling as humans mingle with stegosauri, parasaurolophuses, triceratops and their like. But envelopes need to be pushed, profits boosted. A sensational new arrival is called for, and Pratt's character Owen, an ex-military man turned animal behaviourist, is asked to advise on security. Does it all turn out tranquilly ever after? What do you think?

As in Guardians of the Galaxy, Pratt does such a terrific job of running, jumping, shooting, slugging, romancing and that all important quipping, to make it obvious why he is asked continually if Spielberg is going to make him the new Indiana Jones (answer: an embarrassed grin, a "who knows'" shrug and a promise to call if he does). Pratt is a very physical actor, one whose weight has gone up and down over the years before settling on down. Today in London he is fighting fit, having recently completed his first Ironman triathlon for charity. His shoulders are about as wide as the sofa on which he sits.

He has been an action man from boyhood. He was born in Virginia, Minnesota, in 1979, the youngest of three, the family moving with his father's job in mining. They eventually settled in Washington State where Dad became a builder while Mum worked in a supermarket. Christopher Michael Pratt was into wrestling, football and the outdoors. Especially the outdoors. "I was always fishing, or out in the woods, camping, just spent a lot of time playing outside. Running from imaginary dinosaurs," he laughs. I ask what he was like as a child. He thinks for a while. "I was um ... I was ... creative and ... obedient." It seemed an odd combination. A happy-go-lucky child?

"I was a pretty happy kid, yeah. I think there was always a little bit ... There was always a little bit of a ... A little, tiniest bit of darkness in me as a kid but I never really expressed it. You know, part of growing up with my dad the way he was, a very old-school type of guy, strict, disciplinarian, one note, very stubborn - there was no compromise. It was his house, his rules, and he made you stick to his rules." A bit of teasing went on too. "I was a sensitive kid, so I think I was maybe a little angry about that."

Pratt's parents were loving and supportive, and even when he dropped out of college it was no big deal, as long as he was earning. "No-one in my family ever went to college; there was never any pressure to do anything other than pay the monthly bills by working. My whole family has always been pay cheque to pay cheque. I know that was very stressful for my parents but they did a pretty good job of hiding it from us."

It was while working as a waiter that a producer spotted him and asked him to be in Cursed Part 3, a comedy horror released in 2000. Parts in television followed, including The OC, and in 2008 he landed a part in Wanted, with James McAvoy starring. Pratt was impressed by the Glaswegian, then and since.

"Man, James McAvoy. Truly he is one of the nicest actors I've ever worked with. He did a great job on that movie but he was also so kind and normal. Those are the kinds of guys I always looked up to as I was building a career. He was not a prima donna at all. He's worked very hard, is incredibly talented, collaborative, had backbone with the director, had a point of view and stuck to it and fought for it and was right. The director was treating him like a peer because he was. I thought that was kind of cool."

Moneyball with Brad Pitt came along three years later, Zero Dark Thirty in 2012, then Guardians and Lego last year. All the while, Parks and Recreation continued its award-winning run. Altogether, Pratt's overnight success has taken 15 years. But he always thought he was doing OK. From his first pay cheque - $700 for Cursed Part 3 - it all felt like gravy.

"Just not having to wait table, just being able to pay my bills as an actor, for me has always been a complete thrill. I mean that. I did 12 years of TV. I was a working actor. I wasn't a multimillionaire or anything like that but I was making enough money to pay the bills and to live a very comfortable life and provide for my family. It has been many slow steps, each one providing a better and better view, but they have all felt really good. I've never been panicky about it. I even had dry spells where I would go a whole year without working and I just thought, 'That's fine, we'll be all right.'"

Today, after Lego and Guardians, mid-Jurassic World and pre-Magnificent Seven (out 2017, directed by Antoine "Training Day" Fuqua and also starring Denzel Washington and Ethan Hawke), his career is hitting new highs. Crazy highs. The kind of highs that might turn an actor's head. "There are moments that don't feel very real," he agrees. "There are moments where it does feel silly and surreal and you just take a step back and think, 'I never would have imagined this.'"

Moments such as? "The rooms I sit in now with actors or filmmakers. I sat across from Guillermo del Toro on a private jet flying to Las Vegas and listened to him share stories. I couldn't help but feel I was sitting next to Orson Welles." He's back in wide-eyed teenager mode. Even more so when he tells me about a press tour of China he did for Jurassic World. One show he was on was going to be broadcast to 200 million people. Unbelievable, right?

It does not appear to stress him. Much of this equability has to do with his wife, the actor Anna Faris, and the solid home life they have built together since marrying in 2009. "I think we both know at the end of the day we're very fortunate to be on what is such a wild ride, but the ride will be over one day and we'll get off it together."

There is more to it than a good marriage though, and more to Pratt than a nice, cheery guy. To truly appreciate life's brighter days, one has to have known the alternatives. Pratt has.

When Pratt was 15 his father, Dan, was diagnosed with multiple sclerosis. By the time the disease started to seriously affect him, Chris had moved away though the family remained close. Dan died last year, aged just 60. It is one of two seismic changes in Pratt's life, the other one being the arrival of his son. Jack came into the world two and a half months before his due date, giving his parents the mother and father of all frights. I've watched Pratt describe the moment his wife woke him to say the baby was on the way, and what happened next. When he told the story, at a fundraising gala for March of Dimes, a charity working to prevent premature birth, he just about made it through the speech, so we are not going to revisit those days again today. Enough to say that his and Faris's story had the best of endings. Jack, three pounds at birth, will be a happy, healthy three in August. Pratt acknowledges the two experiences, losing a father and becoming a father, have changed him utterly.

"I always had this philosophy of 'be a free spirit, have fun, don't take anything too seriously'. Then I realised that was because at that time I could afford to not take anything seriously. But some things in my life now I have to take very seriously. I've entered this middle chapter of my life where I'm now not only suffering great losses but also knowing that I'm responsible for the happiness and life of another human being. It feels great. I feel mature, but I also feel like something is filling up inside me that I didn't quite realise was empty."

With The Magnificent Seven now filming, and another Guardians on the way, Pratt's screenplay writing (he has written several) and ambitions to direct might have to be parked for a few years, but one has the feeling he won't forget they are on his to do list. The nicest slacker in the business is just getting his groove on.

Jurassic World (12a) is in cinemas from Friday.