For interior photos I generally first look through all of the rooms and decide the likely vantage points that I'll shoot from. If the shoot is going to be in the daytime and there are windows, I have to decide if it is best if the sunlight will be coming into the shot through the windows, or if the shot will be better without the sun coming into the scene.
It is very easy to use SunTrajectory.net to determine either of these because I can simply stand near the window and hold up my smartphone to show a view through the window and see if and when the sun will shine through it. Should I need to be more exacting, I'll switch to the "Shadow" feature, stand right in front of the window looking back into the room, and see what time of day I throw a shadow and where it will fall. That way I can predict what effect the sun will have on the room at a specific time of day.
For landscape and cityscape photographers, SunTrajectory.net also can show the path of the moon in case you want to include that in a photo at a specific place in your scene.
So I highly recommend this software to all photographers and the only thing that would make it work better for me is if all of my clients had it and did my planning for me. Now if only SunTrajectory.net could also get the clouds to co-operate they'd really be on to something."