Well, it all depends on what the script says. When the director gives you your cue, it’s time to sing.
I’m not a singer (instead one of those unusual people who has perfect pitch but can’t sing for a dime) but I am a whistler. I’ll whistle around people, but not if I’m sitting right next to them or if I’m in a quiet room with lots of people. That’d just be annoying.
I do burst out into song when I know I’m by myself, but occassionally I get caught. Just the other day, I was belting it out in the shower, forgetting that my boyfriend might possibly hear me if he went into my bedroom. He did and later asked me if my shower really was “hella good”. :o
Well, I am a (semi-professional) singer, and I almost always have a tune running around in my skull just itching to burst forth. Since spontaneously breaking out into song is frowned upon in most public places (especially my office), I’ve cultivated the habit of whistling instead (and I am a VERY good whistler, if I may be so immodest). I have also learned to whistle under my breath so as not to disturb neighbors. Singing under my breath just makes people think I can’t read without moving my lips…
Barry
I sing when I walk. It’s a little like a conversation–it helps to pass the time–and also to a certain extent helps me know how far I’ve gone. When I was a kid I used to sing Gilbert and Sullivan operas on my way home from school (long boring walk), and I learned more or less how far I could walk on a given song.
I don’t sing loudly unless I’m somewhere like a lonely beach. I doubt if you could hear me from more than a few feet away. What I don’t understand is why some people (always male) feel compelled to burst into loud, pseudo-operatic mock singing when they hear me. Obviously they’re trying to embarrass me, but it seems awfully silly. And they make themselves a lot more conspicuous than I am.
Wait a minute, are there people that DON’T always have a song running through their head?
My favorite time to sing is while I’m doing my German homework. I sing in English, and it somehow keeps me from trying to word-for-word translate the passages instead of just gleaning the information naturally.
When I’m walking places and I’m bored, I do sing quietly to myself, although I generally stop if someone is around. I also do little dances - generally tap breaks.
I’m another who always has a song running through his head, and occasionally, it spills out.
I once had a warehouse job. For eight hours a night, I’d stack cases of empty beer bottles on skids, then use the forklift to take them to the loading docks. It wasn’t unusual for me to sing while I worked; it helped me keep my sanity. And a few co-workers said they enjoyed hearing it when they came into my work area.
I like to sing. In my life, I’ve been part of the church choir, and a karaoke host, and taken part in amateur theatre musicals, and done a capella doo-wop and traditional tunes with a few other guys at open mike night. Of course, I like to stick with things I know I can do, but regardless of where I find myself, I’ll sing if I feel like it. (Although quietly, in some situations.) I find singing to be fun.
I’ve got to agree with the last two posters. I know that people function without music. The thing is, to me, it’s about as easy to understand as functioning without oxygen.
One of the things that got me tagged as wierd when I was a kid was singing on the bus. I don’t sing in public much now, but I will with the right company. I do so because it fills me with joy. That or because I get a song stuck in my head or because I can’t resist tweaking some lyrics.
That last one can be dangerous. You see, last year I was travelling across Ohio with a couple of ther music loving friends when I mentioned my mother had given me underwear for Easter. That, I’m afraid, resulted in this, sung to the tune of In Your Easter Bonnet.
In your Easter panties,
Scaring all the aunties,
You’ll be the only Wiccan
In the Easter parade!
No, I won’t say who was responsible for which lyrics, the Wiccans or the Christian – they plan on posting here one of these days! 
CJ
I think I’ll go await my Pitting now.
I always always have a song in my head. If it manages to worm its way out through my lips in public, it either means that I’m in a great mood, or else I’m in such a bitchy mood that I’m too busy trying to keep from killing the next person I see to spare the effort to keep my mouth shut. If I’m also smiling, it’s probably the latter choice. Serious!
Always singing or humming here - in fact, this thread made me start singing “With a song in my heart”.
I also drive my husband nuts (short trip) with the “everything has a song” habit. He’ll start talking about a subject, or say a certain phrase, and I automatically start singing the song that best accompanies it.
I always sing in my car. It’s half the reason of having a car, to sing along.
At home, if the song’s there, you’ve got to sing it.
From time to time, I’ll sing along with low elevator music in stores. Usually softly. Most people pretend not to notice (or don’t notice). But it’s there, my voice isn’t horribly bad (I like the sound of it, at least), and sometimes you just want to sing.
I will, in public (very rarely), just kind of sing to myself. Even more quietly… same reason, just gotta sing.
I don’t sing at work.
Helena"When I was a lad" is not easy to whistle so that it’s still recognizable. I’m impressed.
Proven fact; singing along with the music makes the song better. Opens happy receptors which spread enjoyment molecules throughout your woojiwhatsis. The only thing that can increase the pleasure derived is wiggle-butt dancing along with the singing. Knowing this, how can a person [i[not* sing along?
I love to sing, and I am not half bad, but I am incredibly shy about it. I’m working on getting over it. 
I love to sing, and my voice isn’t too bad. But half the time (ok, more like almost all the time) I’m in the wrong key. That doesn’t bother me when I’m in the car all by myself, though :).
I sing all the time. Sometimes I catch myself with people watching, and I’m a bit self-conscious, but usually I continue anyway. It’s not something I can really control very well anyway.
Also, you’d be amazed at how much fun it is to sing with a group of people. Once, while waiting in line at an amusement park, a group of us decided to sing some songs we all knew. By the time we got to the front, we had about 20 other people singing “American Pie” with us. It was a blast!
I like to sing and will usually do so if I hear a song I like. In public, you normally can’t hear me, but I’m sure you can see me mouthing words and nodding in time. At home or in the car where I have no chance of having strangers hear me, I usually sing loudly. But only along with music. I tend to be off-key without something to base the pitches from and I can’t stand to listen to myself.
Out of curiosity, does anyone else sing along with instrumental music? Not with words, but with appropriate sounds? I love doing that. No whistling, though!
I sing quite often, whether there is music playing or not. If there is a good song playing in the store, on the radio in the car, or even on the bus I’ll hum the song in my head.
I don’t mind it when other people do it. People who are great whistlers are wonderful to listen to. One of our bus drivers is quite good and I always enjoy the trip into town with him.
Hubby often sing aloud as well. I love it when he looks me in the eye and bursts into song. So sweet
IMHO life would be much more pleasant if it were more like a musical. People should feel free to burst into song whenever it seems appropriate, and others should feel free to join in, with dancing. We have a huge collective musical repertoire which should be exploited. School children should be taught simple choreography to common songs so that we could stage spontaneous public musical productions.
Here’s a story I love. It may be apocryphal, but here it is anyway:
A man and his daughter were walking down the street. The man started singing. His daughter begged him, “Please daddy, don’t sing. You’re embarrassing me.”
The man was Billy Joel.
I sing or hum pretty much constantly. In the car, on the road, walking down the corridor at work… One of my flatmates’ boyfriends once said to me, “People who hum are sexually frustrated.” Someone else said that humming was a sign of self-contentment, and that I was obviously very happy with myself.
Both are fairly accurate.
I can’t explain why I do it. I just MUST SING. Sometimes I even find myself muttering to myself in public. I’m not ashamed of it. Hell, most of the time I don’t even bother stopping.