My friends are so used to me having a camera on me, and pointing it at them, that they no longer pose. Just take pictures constantly. Eventually they'll forget about the camera. I wouldn't worry about "wasting" film. Sometimes the posed pictures come out pretty entertaining (art, no, fun mementos, yes).
I couldn't agree more.
I like to take people as they are, and photograph them talking or doing things. Eventually they will almost forget you have a camera. Almost.
If it's people you know and will shoot again, give them prints. If they are good they will be happy for you to shoot em again.
Or if I have to shoot a portrait quick, and you are getting that tense stare back from the subject, I'll set up the camera, hold it still and move my eyes away from it, maybe pulling a thoughtfull expression or something. The subject doesn't think you are going to take the picture, they relax that wee bit, then hit the button. People try to anticipate when you are going to shoot, if they are really trying hard to do this, I'll make a bit of a game of it.
A famous photographer, who's name escapes me, talked of wearing them down. He said, I keep going till they are almost bored, and wondering what the hell I'm doing, you see a gradual change in expression.
I'm not suggesting you do that, but you can see the logic in it, and his portraits were wonderful. If only I could remember the name!