I run away (this guy might be dangerous)
I don't do anything (better stay out of other people's business)
I start shouting "Police ! Police ! Stop the thief !"
I step in front of the thief and tell or gesture him to stop
I feign indifference, then trip him with my foot as he passes by
I feign indifference, then extend my arm to his neck when he gets near to knock him down
I get ready and punch him in the face
I take the gun I always carry on me and shoot him between the eyes (I hate thieves)