What happens if I set a bhvr that allows an agent to be interacted with, but I don't give that agent a corresponding interaction script? What does the creature experience when they interact with such an agent?
It depends on the script in question. A C3/DS agent cannot have a BHVR that includes edibility but no eat script, for example: the agent will error when the user attempts to inject it, well before any creature could try to interact with it.
It may not seem that way, but this is true for all script types; I actually discussed this with AquaShee a while back and he was able to determine that scripts that appear to be optional, such as the pickup script, only appear to function that way because the game has a hidden "global default" pickup script that it uses if there is no pickup script for a specific agent.
So really, there is never not a script to match an agent's BHVR, either because it causes an error or because there really is a script involved. A creature that interacts with a "scriptless" agent experiences whatever stimulus exists in the default script.
The only time I can think of when a creature might receive literally no stimulus for an action is if an agent's creator does write a script for a certain interaction, but does not include any stimulus. CFE breeds appear to feel disappointment and/or boredom when an action provides no feedback and will eventually give up, but non-CFE breeds don't have this feature and may continue to try to perform the same action until they exhaust themselves.