That's a good one (and fast), Synth.
Okay, here's my attempt. It's helpful to break a sequence down by motivation (Ollie Johnston asked, "What is the character thinking and why does he feel that way?")
So this scenario might be a girl fixing her makeup before a date. I'd break it down into 3 phases of motivation/emotion:
1) "Can't get my hair right!"
2) "That's much better."
3) "Not bad, girl!"
This translates to 3 key poses. So I hold each key pose for 6 frames and add 10 frames in between for breakdown/ease in/anticipation into the next pose.
I won't necessarily match the expressions or timing of the original frame for frame. I only use it as a guide. My goal is to make each expression read well.
So, here's my example (red is key pose, blue is the breakdown):
Admittedly this could be improved a lot, but the main thing is the strategy in approaching a challenge like this.
