Clark Gregg has been part of the MCU from the very beginning thanks to his appearance (which was originally much smaller) in Iron Man back in 2008. He didn't have a huge part to play in the movie as Coulson, and nobody could have guessed that the MCU would someday come to span 21 movies (and counting) and several TV shows. He went on to star in ABC's Agents of S.H.I.E.L.D. starting in 2013, and he has now revealed the challenging thing about playing Coulson for all these years:
There are parts of him that come very easily to me. His organizational, leadership thing—that feels comfortable for me to slip into too. There’s a protecting one’s loved ones and family, that feels natural. But there are ways that his dedication to duty, and his abilities to be serious and mature can be challenging for me at times. ... At this point, the membrane between us is very fluid. Sometimes, one of my castmates or bosses will come to me and be concerned about where I’m at emotionally, and it takes me a second to realize that it’s me, I’m still in the moment and I’m worried about the person Coulson’s worried about.
Phil Coulson has been such a huge part of Clark Gregg's life over the past decade that his comments to io9 shouldn't come as a huge surprise to many. He's able to slip into the role, which must be convenient for a character that has been called upon to appear in such a variety of projects. Whether he appeared in the MCU films, made the jump to ABC for S.H.I.E.L.D., or went on a blast to the past for Captain Marvel, Coulson is a familiar character... to a point.
Unlike Phil Coulson, Clark Gregg has not been tasked with making life-or-death decisions for the people he loves when facing off against aliens or unfriendly Inhumans or any other baddies that have appeared on S.H.I.E.L.D. over the years. Coulson has to be serious about his job almost all the time, or people can die. If Gregg isn't always 100% serious about his job... well, fans can get some fun bloopers out of it!
Considering Clark Gregg's comments about how he can get caught in the moment Coulson is caught in, I have to wonder how S.H.I.E.L.D. Season 6 felt different for him. He has confirmed that the character he plays in Season 6 is not the Phil Coulson that S.H.I.E.L.D. fans have come to know and love, as original Coulson presumably died between the end of Season 5 and beginning of Season 6 due to his deal with Ghost Rider.
The new character definitely doesn't seem as friendly as Coulson 1.0, and the agents may want to be careful around him. Just because he has the face and voice of a dear friend doesn't mean he's to be trusted. Although there may not be anything to the Skrull theory about how Clark Gregg is playing somebody who looks exactly like Coulson but isn't Coulson, something obviously happened to bring this new character into the mix.
Fortunately there's a new leader at S.H.I.E.L.D. who should be able to help guide the good guys, and Daisy's new duds as Quake bode well. Hopefully this new character of Clark Gregg's doesn't do anything to mess with Simmons on her mission to recover a version of Fitz that hasn't been killed. Fitzsimmons have had to deal with enough misery over the years without throwing in a lookalike of their beloved mentor to mess with their heads!