Interactions with Two

So I'm having trouble writing this one scene. I have my lead characters interacting, Greg and Diane. Diane is attracted to the Greg but won't make a move because she's already in a relationship with someone she is committed to. Greg might be interested in her but he isn't sure he should pursue Diane especially while she is in a relationship. Now I know this type of thing might come up often between people. But having never been in a relationship I'm missing out on the personal experience it is to be in a committed relationship with someone you love, yet have feelings for another. Are there any ladies out there with these kind of situations who would like to share some insight on the matter? What kinds of feelings do you have? Do you act upon them? Do you try to be honest with both guys? Leave comments with your opinion or your take on the matter.