I notice too that TWiT, one of the most popular podcasts on the iTunes Store (loosely referred to as the Billboards for Podcasts) have the guests do voice impersonations at times. I think this has more to do with how Leo Laporte can infect everyone else with his voice acting.
When I used to direct plays, a technique I made use of to “unleash the inner actor” would be to have the characters do a dramatic run through of the play with the lights closed or with something to cover their eyes, like a paper bag … or cucumber slices.
Statistically speaking, hiding behind a mask helps remove inhibition, allowing characters to play their role more effectively (Batman and other superheroes for instance hide behind a mask to transcend into another personality).
Try podcasting in the dark or with your eyes closed. It helps.