I could see Lindsay getting too close to a case (as in 'Stealing Home') and Flack being concerned, talking to her about it. He doesn't want her to end up like Aiden. So he tells her that he cares about her, doesn't want her to get hurt. Only it comes out in a way he hadn't intended, and now Lindsay knows, or is coming to know, how he feels.
That would happen at the lab. Then we have the post-case scenario, where Lindsay (being a stubborn country girl) would be all, "You were right, okay? Now go away." And Don would be all, "Marry me, pls."
But another scenario I've been toying with, is Flack and Lindsay totally butting heads on a case. He thinks one thing, she thinks another. They just clash. And it turns out Lindsay's right, and the perp nearly got away with it -- but she convinced Flack at the last second. So, at the end of the day, he buys her flowers.
Lindsay: What's all this?
Don: Nothing says "I'm sorry" like a hundred dollar bouquet, right?
Lindsay: Oh, Don. You didn't--
Don: Yeah. I did. You were right, and I was wrong. Had to do something to say I was sorry. Sullivan's in an hour, remember.
...
Danny: Nice flowers, Montana. Boyfriend?
Lindsay: Flack. He was saying sorry.
Danny: *snerk* He dated this girl once, for a year or so. He didn't once get her flowers.
(( and basically i love writing out these little dialog scenarios, k? because it's almost like fanfic and it gives me ideas.))