They will just talk with you.
I was wearing my t-shirt with a bunch of stormtroopers taking a selfie with Vader and Obi-Wan dueling in the background. One of the FO Troopers walked up to me, looked at my shirt, and said "Impressive"...then he continued on his patrol.
I've heard if you have your lightsaber out away from Savi's, they will escort you away and you get a cool photo op.