Question about mix bus compression

I'm aware of the benefits of using some gentle compression on the mix bus to provide a bit of "glue" to the sound and make everything work together a little better. I'm definitely up with that and can hear the effect it has very clearly. 
 
What I've always been confused about is this: what about music which has quiet parts and loud parts? In slapping a compressor across the master bus with a certain threshold level, the amount of compression during a loud part is obviously going to be a lot more than it is during a quiet part. But what if, during the quiet parts, you wish to apply the same "glue" (I kind of hate that word ) to the instruments? Is it commonplace to automate the threshold so that quiet parts get a little love too? 

For me, the glue on a master bus is reverb.  That is, a room or small hall reverb set so quiet you can't consciously hear it but loud enough that your subconscious places all the instruments in the same "space".
 
I generally do this by having a "Master reverb" bus with a send on all tracks so I can vary the level of reverb for each track and vary the overall reverb effect with the bus fader.  You would normally have multiple reverbs across different tracks set much higher which will mask this "Master reverb"
If you need a track to jump out at you, automate it's send to reduce the Master reverb level.  If you've got the levels right you shouldn't hear a change in reverb (you shouldn't hear the reverb at all) but the track will detach from the rest of the mix.

Interesting ideas and concepts here. I guess I'd never really thought about using a master reverb before, or using exciters as glue. I've been demoing The Glue by Cytomic and have been very impressed with the effect it has on the master bus with about 2-3dB of gain reduction. Previous to that I was using Native Instruments' Solid Bus Comp which is quite decent, but certainly has a different character to The Glue. 
 
It's good to hear that some people are automating the compressor threshold though. I had thought "why not" but wondered if other people did it. 
 
 

I often use a reverb on a bus to put (most of) my instrument in a room, so to speak. When I say reverb, I really mean early reflections, mostly. In fact, sometime I simply use a delay. It's there to be felt, no heard.
 
It's something I've always done instinctively - it made sense to me. The principle was a good one but not the application -  I used way too much. My mixes have improved dramatically when I started using very short reverbs and started focusing on pre-delay/early reflections instead of the tail. 
 
As for mix buss compression, most of the time I pick a compressor based on what it does at unity, with no compression at all - like IK's Fairchild Model. At any rate, there's rarely more than 1-3 db of compression on the master buss, but even with no compression registering, bypassing the plug-in makes a world of difference.
 
I find that NI's Solid Mix Series works great for certain things, but it's nice to have a couple of options. I've heard nothing but good things about The Glue. I like IK's Buss Comp, the Neve model and McDSP's. Renaissance Compressor often ends up being my favorite - probably the most commonly used 3rd party here. Though I'm still using my bundled compressor most of the time. I'm sure I'd still be using Sonitus if I was working in Sonar. :P

I would agree that most of the time the reverb is very minimal, but I would also say that in certain genres and applications you want to feel like it's in a big space, so I think regardless of the amount used it can be a great way to glue the tracks together.
 
I always use a master bus compressor too Bat. I'm just curious though...are you using a separate compressor bus that goes to master? I have sometimes used both a separate master bus compressor and a compressor on the master. I might send a few tracks to it that I wanted the effect for. 
 
I find that the group function automation really shines for times when you might want to increase/decrease compression or reverb on a few channels that share a common trait. This makes a great way to glue certain parts in different places and make them  flow as one in the mix.

Thanks! 

The compressor's makeup gain will bring up the low levels to some degree, which is another form of "glue" as it produces more consistency in the final output.
 
No one has defined the technical process that creates glue, but sonically, what it sounds like to me is that gentle compression "fills in the cracks" between levels otherwise wider levels.