We're all in that same boat, viewers included. One of the earliest films with that premise that I've seen. The characters were quite memorable, but the German character was the most memorable of all. His ambivalence as a doctor (doing good) and at the same time being one of the enemy and trying to get back to his people was amazing to watch. I went through a lot of back and forth concerning his character and I guess this was all that the movie was about. I'm also pretty sure that the reactions in the audience of the time were much stronger. Hitchcock might wanted to exhort the audience that the enemy should never be underestimated - neither in good or bad.
I've found the "rich" guy a bit too complacent and the card bet was just a bit too contrived to work. Maybe his character didn't feel life-like enough to make such a bet believable. But the dry comments of the lead actress was quite amazing. Her love interest was just a tad too much, but I understand that the story might have needed something to balance out the characters. (again, she might have been a bit too onedimensional in the beginning)