I'm not sure how valuable this is, but I thoughT I'd share: I think one needs to think about/process their physical environs. I work some at a giant haunt park located in Longview, Texas (US). Our resident magician, Brandon...he is clever. The stage (proper) has a few miniature benches just in front, but at the 'back' of the sitting area is a gate/fence. Brandon closes the gate, and stands just inside (very visible)......when customers walk by on the midway, he will chat them up and show them a pretty straight on card trick or two. He THEN opens the gate and invites them inside (so he has built a small tip, to attract attention of other people).....he keeps a table there (in the aisle, essentially) and maybe does a Chop Cup thing or something that gets them reacting. ONLY THEN does he get up on the stage and do BIG STUFF!

The best thing that I came across by experience is that just go with a deck of card in your hand and as you reach the person you want to show the trick to just say, " PICK A CARD" . that's it it's the simple . The thing about asking them that can I show you a magic trick is that you give them an out to say no so by avoiding asking and telling them to do something you overcome that problem . And if they are really busy they will tell you otherwise people who are shy or introvert will just pick the card . And any public place is good . I have reached the spectator in all kinds of awkward situations . And if you want to get a more detailed help just PM me I will be happy to help you.