If you go to the options while playing the drums, you'll notice you have 3 options for fills - Classic Fills, Static Fills, and Dynamic Fills. Dynamic fills should be on by default, but just choose it if it isn't. Dynamic fills are 'random' fills that don't follow the normal song.
While difficulty is not explicitly stated, people are reporting this doesn't unlock on Easy.
Once you've gotten half a bar of overdrive, you'll eventually hit a fill. These are denoted by the sides of the track glowing. The dynamic fills (at least I personally feel) also don't tend to fit well with the song, so you can generally recognize when you're playing them. Just hit 10 of them and you're good to go.