Clark Gregg's Agent Coulson has played a minor role in the Marvel Movieverse, but plays a much bigger one in May's "The Avengers"In 2008, actor Clark Gregg appeared in Marvel Studios' "Iron Man" film as Phil Coulson, an Agent for the Strategic Homeland Intervention, Enforcement and Logistic Division, better known to comic book fans as S.H.I.E.L.D.

Originally a small, one-off character, the put-upon Agent become so wildly popular among audiences he quickly became a staple of the Marvel movie franchises and has appeared in nearly every Marvel film since, even starring in two of his own Marvel One-Shot short films.

With the upcoming Joss Whedon-directed "The Avengers," in theatres May 4, Gregg's role has been expanded to a main cast member as the sarcastic Coulson plays a pivotal part in the film, helping bring the Avengers together and trying to keep them from breaking apart.

"He's got a lot of stuff to do in it, old Agent Coulson!" Gregg laughed during a one-on-one conversation with CBR News about his role in the biggest Marvel movie to date.

A writer and director in his own right, Gregg is as shocked as anyone that his turn as the non-powered Coulson has become so wildly popular. That shock has carried him forward through the past five years, only to reach new heights when he received "The Avengers" writer/director Joss Whedon's screenplay and realized what a large role Coulson would play in the film.

"You work really hard and you do stuff for years and you struggle and you play this part and that part and you're not the star of the movie, but you get fun stuff to do and sometimes when you do get something you try to make the most of it, and in your mind you think, 'Maybe this time. Maybe this character, I feel like they could do more with this character, maybe they will,'" said Gregg.

After a pause, Gregg threw his arms in the air and laughed. "But it never, ever, ever, ever happens!" Gregg said. "So that was the dialogue I was having with myself when I was deciding whether to do this part in 'Iron Man' one which was really just a couple of lines, because the problem with having just a couple of lines is that's the first thing to go, so it just normally leads to heartbreak and it so rarely leads to the other thing!

"[What] Agent Coulson had become in terms of the import of this particular story, and how important his job is in bringing the Avengers together, it kind of felt a little surreal, like somebody was playing a prank and that wasn't the real script," Gregg continued. "But it wasn't, it was the real thing, I got to show up and do that stuff, and it felt like such an amazing payoff to what the journey had been and the fact I had been doing it for five years."

Though the movie does not delve into Coulson's "origins" the way some fans speculated it would, Gregg told CBR he felt the movies fills in many of the blanks in Coulson's back story.

"It made acting it a lot easier because I cared so much about the guy, and some of the stuff Joss had written to me really revealed in subtle ways where his involvement with S.H.I.E.L.D. came from, his fanboy obsession -- kind of a macho sarcastic version of it -- with the secrets S.H.I.E.L.D. keeps and the fighting S.H.I.E.L.D. does to keep us protected from things we can't even imagine," Gregg said.

Gregg as Coulson with Chris Hemsworth's Thor aboard the S.H.I.E.L.D. helicarrier

"As a fanboy it was kind of beyond what I could get my head around," the actor added.

Gregg, who has also appeared in television shows such as "The New Adventures Of Old Christine" and directed the 2008 Chuck Palahniuk adaptation "Choke," stated he slowly became aware of the popularity of the character, the full impact of Coulson's status hitting during a trip to Comic-Con International while promoting the Marvel movies.

"It was the second time when I went to Comic-Con and I tried to do what I always do at Comic-Con, which is buy issues of comics I don't have, and I had to be surrounded by a security detail because there were people dressed as Agent Coulson sort of mobbing me!" Gregg said. "That's when I realized something about the character had taken a hold."

Besides his appearance in Marvel's digital comic "Iron Man 2 Phil Coulson: Agent Of S.H.I.E.L.D.", Gregg said fans have unofficially made and shared their own Agent Coulson art and that he was amazed at the breadth of people interested in the human agent.

"The miracles of Twitter have been amazing because I've really been able to connect with people around the world that there's something about this guy -- that he's a real person, that he's sarcastic and annoyed by these superheroes but also completely in awe of them," Gregg said. "Some of them are artists and have done fantastic fan art, it just becomes a spectacular geek-out thing for me."

Outside of the Marvel movie landscape, Coulson is also a part of the Marvel TV Universe as the Agent plays a role in the new "Ultimate Spider-Man" animated series on Disney XD.

"It's a multiverse!" Gregg laughed, speaking of his cross-media appearances. "In this incarnation he's the principal of the school where Spider-Man and some other S.H.I.E.L.D. protegees are hanging out."

Voicing the character on the show, Gregg thought the differences between his big screen version and animated version were small, and just a matter of writing for the different mediums.

"'Ultimate Spider-Man' is an animated show, it's a cartoon, it's very free and they write fun stuff -- it's kind of amazing because the character has sort of evolved to the point where they can take him and it's like a trope, a different spin on it," Gregg said.

In the first episodes of the show "[Coulson] takes himself so seriously and he takes his work so seriously that they make him the principal of the high school so they can keep an eye on the superheroes under cover, and then he ends up being in charge of the school musical that he takes so seriously that people start to almost mutiny!" Gregg said.

"The Spider-Man musical they do at the high school. Your head can just cave in!" Gregg laughed.

As in "The Avengers" film, where Agent Coulson is revealed to be a card-carrying fan of Captain America, Gregg admitted to being a little awestruck himself the first time he "assembled" with the other actors playing the heroes and villains of the movie.

Coulson's popularity has seen him leap from movies to comics and even animation

"When you walk into a scene and you're supposed to start talking but you look around and find you're talking to really spectacular renditions of Tony Stark and Nick Fury and Agent Romanoff and Captain America and Thor, sometimes it's hard to even get the first word out. But then the Agent Coulson kicks in and he thinks they're all capable of being a little ludicrous at times," Gregg said.

Despite Whedon elevating the character's status for "The Avengers" Gregg said he hadn't discussed the character of Coulson much with the writer/director.

"No, just me dropping of briefcases full of cash," Gregg joked.

Gregg told CBR that the opportunity to work on "The Avengers" was yet another geek-out moment for him as a longtime Whedon fan.

"I didn't know him, I was a big fan of his and I was about to do the panel for 'Thor' at Comic-Con, and this guy came up to me who I quickly realized was Joss Whedon and said, 'Oh , so sorry, I've been meaning to call you -- you have a big part in 'The Avengers,' can I introduce you as part of the cast?'" Gregg recalled. "It was just one more kind of 'Whaaaat? Really?' moments."

Sitting in his catered room as part of Marvel's "The Avengers" press junket, Gregg concluded our discussion by saying, just as he had no idea five years ago this role would allow him to star in multiple films and spin off the character into TV and comics, he has no idea where the future would take him -- as either Agent Phil Coulson or as actor Clark Gregg.

"The short answer is I don't know. It's a journey that always reveals something else. If he doesn't appear in another Marvel movie, as preposterous as that may sound, I would be really happy with how this particular journey of these movies leading up to and including 'The Avengers' really feels spectacular, the way that story and this chapter of the story resolved," Gregg said.

With a grin, Gregg added, "If there's some way in the Marvel Universe to have it continued I'd be thrilled, I rarely enjoy playing a character as much as I enjoyed playing this guy!"

"The Avengers" opens in theatres nationwide May 4.