Think back for a moment. When was the last time you had a good laugh – the kind that made you think, “Oh yes, that's where my ribs are,” or the kind for which the word “hysterical”, a word otherwise reserved for victims of extreme emotional trauma, was invented? What was it that made you laugh? What was the joke?
Let's be honest: it probably wasn't that funny, and your friends probably weren't that impressed when you told it to them.
Starting a musical cue is a lot like trying to repeat a side-splitting joke: it almost doesn't matter whether the joke is actually good or not as long as you get the timing right. I speak from experience, both as someone who writes and as someone who appreciates music that is tied to narrative. If you want your musical accompaniment to sound like laugher and joy, and not a play by play description of your night out with the lads, there are only two ways to enter a scene.
Hide Behind Something Else
The sudden appearance of the words "Star Wars". That moment when Bruce Willis raises his ring-less hand to the camera. Sneaking a musical cue in behind a big dramatic moment is a common pattern that seems to work well. But if you're aiming for something more subtle, you can start on the downbeat of a line of dialogue, or even just a random word. The key is to sneak the cue in behind either the text or some other visual cue.
The first mistake every composer seems to make is to start their cue by slowly fading in from silence. This never works. It's unnatural and it draws attention to itself like a shy, middle school actor shimmying across the stage to avoid being noticed. We all see. We all notice.
There's only one way to fade in without drawing attention attention to yourself: slowly sneak in until you can find something either in the script or on screen to fully come in on, the same way Adagio for Strings quavers at the beginning of Platoon.
Hijack a Dramatic Pause
Yes, CSI: Miami has turned the dramatic pause into a schtick with a schtick, but there's a reason it works. Starting a cue in response to an action immediately causes the audience to reflect on that line.
It's like when someone asks if you were paying attention to what they just said. Maybe you were, maybe you weren't, but by calling that into question they've made sure that practically every piece of your conscious brain is now devoted to their last few words.
A musical cue can do the same thing but with the subtler touch of rhythm and tone. If the preceding action was important enough to trigger a song, our subconscious minds reason, what was so important about that action?
Keep in mind that not all pauses have to be obviously dramatic. The famous Send in the Clowns cue technically begins in response to a quiet confession from the male lead. In fact, many of the best cues have come after lines so subtle you might have missed them if the composer wasn't kind enough to point them out. Sometimes the composer just wants to quietly ask if you're paying attention.