I don't usually like to mix and match but I actually run a website called Everything Voice Over and a while back I put up an article regarding how to use improv in voice over.
There are some really great strategies, whether it be using the in-between moments to hum or think or even outright speaking off script. It can really make you seem unique and interesting. Unfortunately it's not always the best strategy but for certain types of voice over work, it can be awesome!
If anyone is interested in voice over or the improv article. I'll post a link here.
Improv Toolkit for Voice Over
Improv has a tendency to free people's minds up and relax them. It can be a very valuable tool in voice over too!
Keep it up yall!