Theatre Improv - What's the Difference?

My dad grew up in saint louis, but for a time my parents lived in chicago. He saw an audition for a group that was going to be doing imprave,. and he tried out for it. It was run by a man named dell close who is one of the most important founding figures in the modern art of long form improvisation. And when i hear my dad describe what they were doing, a lot of it sounds like an embryonic version of what i would a up doing decades later at the upright citizens brigade.