It really depends on exactly what you mean by "interactively".
Can you or someone else enter something on your online tree and it automatically update FTM- the answer is a resounding NO.
There is no syncing between the member trees and FTM whatsoever.
You have to make a choice between:
1) Do your work on your online tree and occasionall download that tree into FTM (which will create an entirely new file on your computer).
2) Do your work in FTM and occasionally upload that file to Ancestry Member Trees (which will create an entirely new tree).
See the Family Tree Maker and Member Trees message boards for LOTS of discussions on this topic.