Too often we think of ambience as a set-and-forget parameter. You choose the environment you imagine your music to be created in (large room, concert hall, etc.), blend the wet/dry ratio to get it just right, and then you don't think about it. But there are many situations where you can use reverb effectively as a sort of "post-note" event--one where you're not trying to simulate ambience so much as giving the listener an added dimension to the sound that comes after they hear the normal acoustic envelope (attack, sustain, decay/release). It's particularly effective on notes that stop short--ones that have space after them. In that situation, you'll hear some interesting additional activity after the principal note, chord, or sound decays naturally.
To start experimenting with reverb this way, try using something you're familiar with but that gives you some meat to work with. For example, start with large rooms and halls, and try some simple EQ tricks. One of my favorites involves "band processing," where only certain frequency ranges in the reverb get the tweaks. For example, to emulate a harsh spring reverb, you'd select a high-pass filter and boost that treble content even further, even to unnatural emphasis. Remember, you're not using reverb to simulate the environment, so distortion and non-realism are okay here.
Fig. 1: Click for full image.
A good way to tune any frequency-specific reverb is to turn the mix to 100% wet and set the room size to large. That way you hear only the affected frequencies and hear them in an exaggerated fashion. In the case of our high-pass/treble boosted 'verb, we might get a tinny sound, but sometimes the tinnier the better if you're going to be mixing this with a full-range dry signal.
Figure 1 shows the NaturalVerb plug-in from Steinberg (using WaveLab as a host). Using the HPF and LPF controls in tandem, I created a quasi band-pass setup. When I tried the same thing in another plug-in (the Sony Reverb, Fig. 2), matching as best I could the numerical data, I got totally different results. You can see just by looking at the parameters in the NaturalVerb vs. the Sony Reverb that the former would yield more subtle results--which in this case didn't suit my purposes. I actually liked the quality of the Sony Reverb better. So this shows that like anything in technology--and this is true of digital thermometers as well as plug-ins--that just because it has a numerical output doesn't mean it's consistent with other gear giving you the same information. In the end, I found the best solution in Pro Tools' D-Verb, so I based my DAW environment on the plug-in quality. Kind of a backwards way of approaching things, but it worked.
Fig. 2: Click for full image.
Once you have decided on your basic sound, you may want to add it only selectively--as in applying it to certain notes but not others. To do this smoothly, route the "weird verb" to its own track and then bring the volume up and down at selective times, but to this with write-automation enabled, so that the results are almost like an instrument would play--regular, fast, and rhythmic--at least when compared to how "normal" mixer moves are made with a fader (which seems to favor more gradual and graceful introductions and exits).
What's even easier--and more precise--is to automate the on/off status of the effect itself. I could have done this on a single track if the write-automation in Pro Tools included the bypass switch in the plug-in effect, but it doesn't. So I simply routed the reverb-only signal (i.e., 100% wet signal) to an adjacent track and write-enabled the automation as I turned the mute switch on and off in selected places (see Fig. 3).
Because you hear reverb on the tail end of a note, and because the effect's entrance is usually masked by the note itself, the "hard switch" approach of a mute button works just as well as a fader move. In fact, in some cases, it works better, such as when you turn the switch on and off in rhythm, creating "ghostly subdivisions." It reminds me of when Eddie Van Halen used to flick his guitar's pickup switch back and forth in rhythm, where one pickup's volume was completely rolled off and the other was wide open. Only doing it with a DAW is a lot less cooler-looking.
Fig 3. Once the "weird 'verb" is in established and in the DAW environment (in this case Pro Tools), I use automation and the mute switch to have the effect kick in and out in precisely the right places.