First of all, Youngcat, I respectfully disagree with you about a liberal arts Theatre Degree. Although it is a relatively useless degree, right up there with Philosophy when it comes to real world application, I also believe that there are a great many programs out there that allow you access to a myriad of teachings and methods and technique's, and also allow you the laboratory to experiment and fail and try again until you find something that works for you. One thing that Improv and Acting share is that the greatest teacher is failure. Does that make for a great actor? No. But by the same token it does not garuntee a bad one and doesn't necessarily require deprogramming.
I guess it is all in the experience. I am of course refferring to a non-conservatory liberal arts undergraduate degree, just to clarify.
As for the differences, in my opinion there are many. I have often thought of Improv and Acting as different circles that intersect and in turn share space. They are different, but they share a significant amount.
One of the differences for me is rehearsal. If I get close to the mark while rehearsing a scripted dramatic show, with the help of a good director and some homework, I can come back tomorrow and get even closer, until I finally discover that moment. The exacting specificity of the words and the actions frees me to lose myself to an extent and just be within the confines of the character that has been created. I am working on a character, and in a larger sense a show in rehearsal for straight theatre. In improv, I am working on my choices, my reactions, my initiations, and in some respect, myself.
After doing nothing but Improv for over a year, I went back straight theatre for a production of Mister Roberts. After about a week of rehearsal, my director gave me a note that went something like "Brian, when you enter, stay to the right side and don't ever go to the left, and when you get to Roberts, stand there and do not take a step until you exit." It was wonderful, so specific, and it worked and solved the problem the scene was having. It was a small specific director's note that I could tuck away. It wasn't a 'coach' telling me that next time I was in a similiar 'scenario' a better choice might be. . . . AAARGH! No, it was direct. That thing you did? Don't do it again.
I also like having a director, a stage manager, a props person, a costume designer, a lighting designer, a fight coordinator, a fight captian, etc. I like the idea of collaborating with a wide variety of artists as you move towards a final product, as opposed to a team of performers. But maybe that is just me.
And to wrap it up, I love the product. In straight theatre, you plan on putting up a final product, a show (hopefully) that you are proud of and that is worth the price of admission. You have made your mistakes and taken your chances in rehearsal and now your have gleaned the best from the process and are ready to unleash it on the waiting public. During The The Two Towers, a year and a half ago, we had good shows and we had bad, but we were so prepared that the difference between the two was almost imperceivable to anyone who wasn't involved in the process. Consistancy.
Oh yeah, one last difference, then I will stop. I have never seen believable stage combat in Improv. I have rarely seen violence work on an Improv stage, whereas I find it thoroughly engaging and riveting when done well in straight theatre.