The most exciting thing about Lucasfilm’s upcoming Star Wars: The Last Jedi is Luke Skywalker returning to the saga.

[Warning: The following may contain spoilers for Star Wars: The Last Jedi.]

Skywalker may have returned to the series in Star Wars: The Force Awakens, but those brief seconds audiences glimpsed of the jedi standing on a cliff of Ahch-To didn’t tell us anything about his character. It’s been decades since Skywalker has been seen and, after everything he’s been through, it’s fair to say the character has undergone some changes.

In an interview with the AP, actor Mark Hamill said Skywalker underwent the biggest changes, more than anyone else from the original trilogy. Hamill added that because he didn’t get much screen time in The Force Awakens — a decision he understands — it’s vital that The Last Jedi explain what happened to the character in the years that passed. This backstory, Hamill added, will help explain to audiences why he seems so different from the version of the character they remember.

Luke changed, I think, more than the other characters in the original trilogy, from callow farm boy to a Jedi in training to finally a Jedi master. What you're talking about is what really fascinated me. Between "Return of the Jedi" and "Force Awakens," there's just decades of history that's unknown. So I was wondering how they're going to handle all this. Now, obviously, with "Force Awakens," J.J. (Abrams) had a full plate. It was inevitable that they would push me down the line. I was sorry I wasn't able to work with any of the original actors again. But it's not about us anymore. In this new one, I was saying to Rian Johnson: I need to know my backstory. It was kind of unclear. You read where he is now and what he's doing now and sort of have to fill in the blanks for yourself. So I did do a backstory myself. It's not about Luke anymore, so it's not really important. But I had to make sense of it for myself. What Rian came up with, I was stunned.

Hamill’s quote isn’t the only proof we’ve been given that Skywalker will have changed in the years since we saw him in action. The actor posed as the character for a German magazine. In the photo, Skywalker is wearing all black and the red light in the bottom left hand corner has people theorizing that Skywalker may have changed more than anyone has led on.

Director Rian Johnson has spoken about Skywalker’s role in The Last Jedi before, confirming that Luke’s decision to abandon the jedi and disappear to Ahch-To is an important question he wanted to explore.

“Figuring out where [Luke's] head was at was the very first thing I had to do when writing the movie," Johnson told Entertainment Weekly. "I had to crack this. And it had to be something for me that first and foremost made sense. Why did Luke Skywalker go off to this island? That was the starting point, and that's what the entire movie explores."

Star Wars: The Last Jedi will be released on Dec. 15.