When clicking on the "Live Assisted" option to chat with an expert, it says you have to agree to let TurboTax share your information. Does the expert have full access to my return and all my personal information?
No, the agent will do a screen share with you so he or she will see what you are looking at on your screen. You will be in control of what the agent sees, if he or she needs to look at something on your return, you will navigate there and the agent will see whatever you are seeing.
Do I need to install something to do the screen share?
No, you don't have to install anything to do the Live Assist screen share.
So this isn't entirely true, I ended up doing the screen share, but before doing that, the person was able to look at the "back end data" seems that they have access to all the data in the return. The screen share seems to just be for them to guide you through what to click on.
What backend data did they have?
In general, no one can see anything prior to you sharing your screen. Since you are using live assist, this may be a feature that they can see a small bit of information or it is also possible that they were answering based on general knowledge or a pattern of things customers do, like we do here in the Community. Example, if someone asks why is my refund changing when I entered the second W-2, there are usually only so many reasons this will happens so we will answer with those reasons which usually resolves the issue for the customer.