Film music tends to be sectional. A cue will be happy for a bit, then turn sad, then anxious. It’s the nature of the beast. Sometimes we want the changes to be abrupt, but sometimes the movie calls for a more invisible transition. I like to call this dovetailing. Just like in woodworking, dovetailing requires a carefully constructed overlap of two sections. Following is a quick tip for smoothly moving from one segment of a piece to the next.
Rather than allowing one section to end before beginning the next, start introducing elements of the later part into the first or vice versa. If your first bit has an eighth-note feel and your second is mostly whole notes, begin softly bringing in whole notes under the end of the first section. You can also fade the eighth notes at the end of the first part or carry them softly into the second to create more of a bridge and smooth the transition.
In my experience, dovetailing mostly involves the accompaniment. If one section uses arpeggiated figures in the background, carry those over into the other segment. If both parts use arpeggiation but of different types, change the arpeggiation before or after the section change. It’s really just a matter of gradually altering the accompaniment rather than changing it suddenly. It’s also easiest at first to focus on the rhythm. A triplet feel can slowly give way to a quarter-note pulse by gradually replacing triplets with quarter notes until you arrive at your destination.