Ah I see, you are going for the Sims / RPG style thing. I think semi-transparent would probably look the best? You could of course create another image to display in its place on mouse over instead, or if you prefer or you could hide it entirely.
Ok so if you are wanting to be able to interact with things hidden behind a wall then you are going to need a condition to determine when to enable said objects. On mouse over the wall go to scene actions > set object visibility. This will allow you to specify the opacity level (transparency) of an object as well as set the amount of time it should take to transition to the new opacity value. A value of 0 would be instant, whereas a value of 1000 would be 1000ms which is 1 second.
Next you would create an action for setting the condition we talked about earlier to true. This would enable the objects / action area behind the wall so that the mouse cursor can detect it.
For setting it back you would use a on mouse out action & just do the same actions again, but would set the condition to false & the opacity back to 100%.
An alternative method you could use would be to create action areas in the way system of the scene, so that when your character walked into a specified area you could set the opacity of the wall to make it invisible or semi-transparent & enable the object areas for interaction & do opposite for when character leaves the action area. Personally I think this method would be much cleaner.
Sorry for the epic amount of text. Hope this provides you with a few ideas.