A rail manipulator indicates the possible range(s) of movement of a part of a computer-generated character in a computer animation system. The rail manipulator obtains a model of the computer-generated character. The model may be a skeleton structure of bones connected at joints. The interconnected bones may constrain the movements of one another. When an artist selects one of the bones for movement, the rail manipulator determines the range of movement of the selected bone. The determination may be based on the position and/or the ranges of movements of other bones in the skeleton structure. The range of movement is displayed on-screen to the artist, together with the computer-generated character. In this way, the rail manipulator directly communicates to the artist the degree to which a portion of the computer-generated character can be moved, in response to the artist's selection of the portion of the computer-generated character.