Goodman explained: "I'd just kissed it goodbye a long time ago. I never figured I'd be in a war movie."
The 'Monuments Men' actor - who plays one of the soldiers helping to recover art stolen by the Nazis during WWII - believed he had come too far in his career to be cast in a historical military film.
However, the 61-year-old star admits he had a lot of fun working alongside George Clooney, Jean Dujardin, Matt Damon and Bill Murray in the comedy, but he struggled with the costume at times.
He said: "I'm very grateful to have been asked and it was so much fun to put on a uniform every day. Except for the boots, I had trouble with the boots.
"But it was like playing. It was like being 12 years old again, out in the yard."
Goodman insists the cast got on really well and Clooney - who also directed the movie - was always cracking jokes in between takes.
He explained to the Belfast Telegraph newspaper: "We all had great respect for each other, and once the cameras stopped rolling, once he finished his shots, then we would laugh, until the next set up. We just sat around telling jokes, swapping lines and just having the best time."
Tagged in George Clooney John Goodman Jean Dujardin