It’s not uncommon at all for someone to hand their iPhone to another person. Albeit a temporary action, this happens when someone wants another person to view a photo or video, try an app or game, or assist them with a troubleshooting matter. As common as this act is, a legitimate concern is what the person will do with your iPhone after you hand it to them.
If you’re among the group of people who get paranoid about someone rifling through your personal data upon handing them your iPhone, and you're certainly not alone, then you’ll probably come to appreciate the likes of a newly released and free jailbreak tweak dubbed ScreenFreeze by iOS developer Greg0109.