When your walking and your boyfriend puts his arm around you what do you do?

Well, you could put your arm around his waist, or maybe put your hand in his back pocket.

And if you didn't want to do that and only wanted to hold his hand or something, you could, stop and face him, his arm will probably drop to your waist, and you could say something in almost a whisper like, "..you know you mean a lot to me right" and look into his eyes, or you could just kiss him, or both, then take his hand and walk away :)

Hope this helps, and Goodluck! :)