“Look, that whole episode was hard, and also because I did go through and smack everybody with Lucille at a certain point. Everybody took a hit. All of that was hard. It got to the point where I didn’t want to do it anymore. … Emotionally I was completely drained—all of us were, I would imagine.
Andy and I just went through the ringer. It was a hard episode, and having to get there time and time again to do these horrible things… They’re all good people, and I love them—the whole cast—so to keep riding them as hard as I was riding them, in-between takes it was like, god, you’ve got to catch your breath a little bit. It was just so fucking heavy at all times. In the show that aired, there’s no let up, and it was like that for 10 days for us. It wasn’t just 40 minutes of it; it was 10 days of that, every day, all day.”
Rick truly fell for Michonne post “Well. Must’ve been something else then,” and before he left Hershel’s ass hanging so he could run to the gates for Michonne. It was during that hiatus. And when he finally works up the nerve to ask her out, so to speak, she turns him down (albeit not on purpose), and then the Governor shows up and everything is shot to shit.
Rick was into Michonne pretty much since the beginning. The proof’s all there. Now, when did he realize it was love?
The moment when he saw how truly sad she was; when he decided to make everything right, for her. Even if that meant going another 500 miles, or 1,000 miles on a hunch. He does that for her, and only her, because he’s a man in love.
Now. As for Rick’s flashback, he thought of some of their happier times, which included her playing with Carl, seeing her again after the prison, making her laugh in bed, and seeing her smile, looking at him. Those, in my opinion, don’t point to any first time, just that they were some of the happier times of the many moments he has shared with her (also, the footage that they had on hand).