And yet: The man can't stop eating jelly beans. Or giggling.
"I'm soooo sorry. So sorry," he says smiling, with a red one clenched between his teeth.
"I haven't eaten jelly beans in soooo long, and I love them so much."
"And they're in these little things, you know what I mean, to make them look appetising," he continues.
The actor's disarming enthusiasm for sweets is almost enough to make you believe him when he says things in his world aren't much different from the way they were 12 years ago when he was a senior at Washington's Georgetown University, a few blocks away.
Almost. But let's not forget that this is the guy whose bright blue eyes proved so seductively loathsome in Wedding Crashers and She's Just Not That Into You.
Whose acting resume includes stints on Alias and Nip/Tuck, and a juicy role in Jim Carrey's Yes Man.
And he's the guy who leads the ensemble cast of what could be one of the year's highest-grossing comedies, The Hangover.
He has returned to Georgetown to promote that movie and is ready, now, to tell us what it's about.
"It's about jelly beans," he says.
"No. It's really simple. It's about these three guys who take their friend on a bachelor party to Vegas for the weekend right before his wedding.
"They think it's going to be this big party, but they wake up in the morning 10 minutes into the movie. ...
"There's a tiger in the bathroom, there's a baby in the closet, they don't know where Doug, their groom, is, and they can't remember what happened."
Cooper (34), wanted to make the film as soon as he finished reading the script. No, it was more than that: he knew he was going to make it. Which is just the kind of guy he is.
That brings us to a quick diversion and a message Cooper wants to send to one lucky lady out there.
Jessica Alexander, are you listening? You're the reason he's here. Er, was originally here - in the neighbourhood, anyway.
You see, Cooper and Jessica attended the same Philadelphia high school. She was a senior; he was a few years younger. The day the yearbook came out, he saw it: "Jessica Alexander, Georgetown University".
"And I just liked the way it looked," he recalls.
And just like that, he made up his mind to go to school there, too.
"That's how I am in my life: I get something in my head, and that's it," he says.
"Like The Hangover was an example of, `I just feel like I'm going to do this movie', and I ended up doing it."
Eventually. After reading the script, Cooper met director Todd Phillips, who was also responsible for the buddy movie Old School.
Then for a year after that meeting, Cooper didn't hear a word about the project.
"I thought it was dead. I was up in Williamstown doing a play. I got an email from (Phillips) saying, `Are we going to do this, or what?' And I was like, `Do what?'
"The next thing you know we're shooting the movie."
It has been a big year so far for Cooper.
Aside from He's Just Not That Into You and The Hangover, he also has a romantic comedy with Sandra Bullock called All About Steve coming.
"Yeah, there's definitely been a progression," he says.
"That's undeniable. ... But everything I've been able to do, my reaction is the same reaction as everybody else's: `How did that happen?'
"But I'll take it."
