It's a large topic, fitting music to picture!
Most Media Music is Recorded to Clicks
Most media music has pre-records these days -- whether it's a student film or a big-budget feature, composers are combining tracks recorded at different times:
- instruments played by the composer at the studio,
- vocal or instrumental soloists, and
- orchestral or choral sections
Moreover, when there is a larger ensemble like an orchestra, quite often media music is recorded in sections, rather than having the full orchestra record all together. In other words, you record strings, then clear the room, then brass, and so on. It is not uncommon to record even separate string sections (high and low, or even greater separation). That allows maximum flexibility in mixing.
To keep all that together, the engineer transmits to players and the conductor an audible "click" through their headphones that helps keep everyone on the beat.
Considerations Recording Live Players to Click
Even with clicks, follow some common practices in your writing so that your players can most easily stay together.
1. Favour meter changes over tempo changes -- In general, it's easier for players to follow meter changes than click changes. For example, it's very hard for the players and conductor to follow click changes for a dramatic Ritard -- where the tempo nearly stops -- and then enter in a new tempo at a downbeat. Therefore, if you want to the music to feel less rigidly "on the beat," it's far easier (instead of a massive Ritard) for players to stay together by keeping the tempo itself constant, adding rubato or ritards or other musical tempo changes by adding an extra beat or extra bar. In other words, imagine you're in 4/4 at a slow tempo and you want the music to hesitate or pause -- rather than changing the tempo, make one bar 5/4 or even insert a 2/4 extra bar, rather than using click changes radically. It's much easier for the players and conductor to follow the meter changes.
2. Big Tempo Changes -- If your piece jumps tempo radically (say, from 89 bpm to 155), ideally it's best to insert a bar at the new tempo while the orchestra is holding a chord. That way the players can hear the new tempo in their headphones, so they can all come in together once the musical pulse resumes. While playing the held chord, they can be doing something dramatic --tremolo, crescendo, decrescendo -- but the crucial point is that the players can hear the new tempo in their headphones for a bar or two and then enter altogether.
3. Subdividing -- If you really want to incorporate a ritard or accelerando or other tempo change that is fairly extreme, you may want to divide the beat in two (or more), so you have eighth note clicks instead of quarter notes (quavers instead of crotchets) for a bar or even for a couple of beats. This can work, but remember that getting everyone to play these tempo changes together can take extra rehearsal time. It also helps enormously if you can have striped video for the conductor, which some DAWs accommodate.