Jimmy Stewart and Margaret Sullavan in love?

I was flipping through TMC's book Leading Couples and one of the chapters was devoted to Jimmy Stewart and Margaret Sullavan.

http://www.amazon.com/Leading-Couples-Turner-Classic-Movies/dp/0811863018

Of course, we all know that they met in college and she went off to Hollywood first and helped Jimmy Stewart break into the film business. There's even a cute story that Sullavan apparently told where she said that Stewart invited her to some sort of dinner party and stated that it was the longest, most awkward, most sincere invitation she ever received.

However, the book claims that Stewart was always in love with Sullavan in real life and never got over her. One of the directors who directed them (forgot which one) was quoted in saying how obvious the whole thing was. He said Sullavan would work very hard in helping Jimmy get the character and they worked a lot off camera. He said that when Stewart would have a scene with Sullavan, he would just light up and something would just change in him. The wife he ended up marrying (at age 41) is even quoted saying that it was obvious that they were in love with each other but that Sullavan was more in love with her career.

After reading that, I thought it made total sense. I mean, I've seen a few Stewart pics, but there really was something different with the way he was with Margaret Sullvan. In the necklace scene in the end (and the sick bed scene), you can tell that a lot of that just wasn't acting. If it was, then that's one of the best romantic acting jobs I have ever seen (if it wasn't, it still is).