Clark Gable needed special exemptions due to his age. Humphrey Bogart served in WW1, but was denied in WW2 due to his age. Which is kinda weird since they're only two years difference in ages. While filming Flight Angles Wayne Morris decided flying was pretty cool, so he joined the navy and flew hellcats with VF15. It very nearly tanked his acting career. Side note, the book Fabled Fifteen about that carrier group is a good read.
I think the Brits had David Niven who took a break from movies, moved back to the UK and rejoined the army at the outbreak.
