When used indoors against a wall how is it secured to the wall?

You will use it as you use it outside, you will need to use the legs. The straps are for the wind, indoor you will not have a problem with that. Another way is to avoid putting the very bottom pieces on the frame then secure it to ceiling grid. You can also turn it upside down and place the legs in the ceiling, but that is more work. You can either use stand or install brackets on wall to hang.

For more information please check out our guide setting up a great backyard setup
http://outdoormoviehq.com/ultimate-guide-to-outdoor-movies
Or take a look at our recommended projectors and screens
http://outdoormoviehq.com/outdoor-movie-screens