I think in theater it demands that you say the same words every night and make it feel fresh and new. Improv demands that you be operating at the highest level of your creativity intelligence. So these two skills are both very important but I've seen people who are very skilled at one area struggle with the other. Either improvisers feel constrained by having to say the same thing over and over again or people who are really good at doing scripted work feel intimidated and exposed doing improvisation.