The movable form is one that exists within each of us and all the beings. Immovable form is one that exists as deity.
In movable form, if we do not believe in God as the inner power, he chooses to stay within us quietly keeping us alive and letting us experience the results of our actions. But, if we begin submitting ourselves to that powerful in-dweller, He is then seen to the world through our actions! We do not become Him, but we will be chosen as His instruments to serve his objectives selflessly, happily with enormous positive energy!
The other movable form is Vibhava form. This is when He chooses a time, place and a form and comes down to this world. Examples are: Sri Rama, Sri Krushna, Narasimha etc. He takes this form to be along with his devotees of that time-period so he could play, eat, talk, and live with them!
When you visit a temple, or pray to the deity at your home, do NOT think that the form is any less capable than God. The deity form is a true incarnation of the supreme power and is equally capable. But, because He chose to accept copper or bronze or other material as his body, he behaves in accordance with the physical properties of that material. He accepts anything you offer through his eyes. He responds to your prayers in your heart!