Yeah, I think your example is dissimilar enough not to be relevant because it would involve rigging and actual electrical work. For that you should certainly have a licensed professional. Any time you are hanging anything over anyone's head, or dealing with electrical wiring (different from extension cables) you should be a licensed professional.
A better example would be if your ID called and asked you to come over and plug a light into a different wall outlet or plug in a light for her (into an existing plug). Would you do that? Yes! But I will first look around to see what else is plugged into that circuit. I have installed Picture Lights over Paintings but that is just plug in. If they want the lights to have wires go into the wall and connected to a wall switch, I would refer them to an Electrician.
Sometimes (even in this day and age) an accident is just an accident. From what I have been told (by my insurance company and a lawyer friend) is that there would be no liability here, and the insurance companies would most likely work it out. As long as there was nothing incorrectly manufactured, no negligence on your part, and the lights were being used for their proper purpose, there is no licensing needed to plug in a light (believe it or not as long as it is not "permanent" there is no licensing needed to hang things over people's head, although if it is not done correctly and fails there will be liability issues). Thus it would be up to the party filing suit to prove any of the above were a cause of the accident. Now you might be sued (as Ben has said, if anyone is hurt, they often sue everyone and sort it out later), but there would be no liability (please note this is not legal advice, just a conversation with a friend in civil law), most likely the venue would file on its own insurance or both insurance companies would work it out between themselves (if there was no injury). You would be just as liable as if your speakers overloaded and burned down the building, but it would not be a criminal matter because you were not a "licensed electrician".
Now to your specific example, if you overloaded the circuit (plugged more than 20A into a 20A circuit which for some reason didn't trip the breaker or overheated your wire and caused a fire) then you are certainly at fault and will be held liable.
But plugging a 1.5A LED light into a 20A outlet? What if there is already a lot of stuff already plugged into that circuit that you are not aware of? No, you do not need to be a licensed electrician. Liability is a different issue than licensing. After all, would you be liable if your speakers overloaded and burned down the venue? Why would lighting be any different? Code wise and liability wise it is not.
Also keep in mind you need to tell your insurance company exactly what you do, so they can provide the proper coverage!!! If you hang lights overhead, etc tell your insurance company. Good Advise.
Mike