Travolta told The Early Show contributor Jess Cagle it was a role he had to play.
"After Sept. 11, I came to Ground Zero. Then I traveled the country and I went to all the firehouses. And I felt a kinship to these men and women," he explains.
"Then suddenly, a script comes that is completely an homage to these people. And if I could be any part of this, I could then feel like I could help ... give back."
Since he felt like this was the first movie in his career that he did for the bigger picture, for the greater good, does that mean that he is changing the way he chooses his projects?
"No," Travolta replied. "I've always wanted to do a picture where I could feel that way.
"I love our country so much. It's like I really love the idea of doing something like an 'Apollo 13' or something that has a patriotic feeling to it. And this movie was the first one that really was presented to me that had that."
The film has an ensemble cast that includes Joaquin Pheonix.
Travolta told Cagle it's not the size of the role that's important: He's built a very successful career by not being a conventional leading man.
"I'm a character actor with marquee value. And I'm glad of it, because I get more opportunities that way.
How does he think that happened?
"Being a child actor in New York, you just know your job is to portray different characters, and to their full potential. ...So I never - this thing called (being) a movie star wasn't an appendage, it was an icing on the cake.
Travolta turned 50 this year and, notes Cagle, has said he never felt better physically.
"Yeah, physically," Travolta smiled. "But - it's just weird. ...I've always had the child inside me. So I still kind of feel 12 or something. I don't feel 50!"
Cagle asked which three roles Travolta think he'll most remembered for by audiences, and said there's no doubt - "Grease." And after that, an array of roles: "I can only say that because everybody I meet has a different favorite part. ...After 'Grease,' everybody's got a different favorite movie."
Cagle observed that "Welcome Back Kotter" is almost 30 years old. How does Travolta think he has fared, professionally and personally, over those three decades?
"Professionally, I haven't changed very much. I was always a very due-diligent, hard-working actor, from the time I was 12 years old. So that was my path, my destiny.
"Personally, I have less ups and downs. I'm much more steady. I feel more balanced in my life than I've ever felt before. So that's a good thing."
Travolta plans to start writing his autobiography in January, and it should be in stores in 2006.