Scripter

The scripter is used to open up the scripting GUI.

NPC
Right clicking an NPC will open the scripting GUI for that NPC.

Blocks
The scripting GUI for the scripted door and block and be opened again by right-clicking with the scripter. Holding it out will also hide any custom texture loaded onto the block so that it can be easily found.

Players and Forge Events
Right-clicking the air will bring up a menu that allows the player to edit scripts for players, or scripts for Forge events.