Please take a look at the short video at this link.
I need to remove the person who walks into the shot.
I have the complete background available without the actors in it, so if I could use a matte to remove the middle area then I could replace the background.
Not sure if, in this case, rotobrush is a good solution, or perhaps some other After Effects technique.
I have After Effects CS5 Master collection. What would you use in your AE bag of tricks to solve this?
This clip is the entire length of the entire clip.
First, the general answer that---when you internalize it---will help you to come to peace with this sort of work:
All of the above.
No real compositing task is ever accomplished with just one feature or tool. It really helps if you learn how to use all of the compositing tools and then hack and carve and whittle away at your scenes with them in much the way that you use various shapes and sizes of tools in a physical toolbox.
OK, now that I've said that, getting to specifics:
You're lucky in that a) your actors are very much differentiated from the background and b) you have a clean plate to put behind them. You're probably going to be able to get 95% of the way toward done by using the Roto Brush tool in very little time. Make a matte with the Roto Brush tool, knowing that it might be a little rough. Then either use conventional rotoscoping or painting on the alpha channel with the paint tools to clean up the areas that the Roto Brush tool doesn't get perfectly.