Regarding God, the switch from “vous” to “tu” is relatively recent. I suspect it dates back from the 60s, following Vatican II. As a little kid, I learnt from my parents “Notre pere qui ETES aux cieux, que VOTRE nom…” but when I joined sunday school, I was taught to say “Notre pere qui ES aux cieux, que TON nom…”
And yes, “tu” is more and more often used. For instance my grand-mother, though of modest origin, used “vous” when adressing her father. What you see in movies reflect quite correctly the evolution, as far as I can tell. As for most other things, a major change seems to have happened in the younger generation in the late 60s - early 70s. Nowadays, most people would use “tu” for an acquaintance, at least if there’s no great age difference. Older people tend to use more often “vous”, not only because they’ve been brought up with different habbits, but also because for some reason, they seem to use it more often as they age (me included). Also, “tu” tends to be used more liberaly by urbanites than by rural people. But honestly, I can’t really give a general rule.
You should give up any hope to know when exactly you should use “tu” and “vous”. Of course, there surely are “miss manners” rules, but mostly nobody is going to follow them. It’s based on a random mix of factors like familiarity, setting, age and relative age, personnal taste, local customs and whatnot. Even in the same situation, different people will make different relatively arbitrary choices. For instance, in my workplace :
-With the boss of the boss of my boss, I would use “vous” and Monsieur Familyname
-With the boss of my boss, I would use “vous” and his first name.
-With my boss and all my coworkers bare one I would use “tu” and Firstname.
-One of my coworkers, with whom I had cordial relations, still got “vous” and firstname for some reason. We both enjoyed the distance it created, I suppose.
-One of my subordinates wasn’t previously a coworker, and used “vous”.
Of course, this could be different for another colleague, and even more so in another workplace.
I tend to say “tu” to a stranger significantly younger than me (say, when asked directions), but significantly more often so when they’re males than when they’re females. There are some vague acquaintances with whom I switch back and forth from “vous” to “tu”. I sometimes deliberatly refused to switch to “tu” because I disliked the person. I might use “tu” with someone I just met in a cafe, while I use “vous” for all my neighbors, even those I’ve known for years. Some teachers will adress 11 yo with “vous” and others will use “tu” with undergraduates. And so on… If I stopped to think about it, I could probably analyze the reasons why, but I don’t.
Besides the obvious cases, like friends or at the contrary a random elderly stranger, there’s a lot of wiggle room. You can even “play” with it. For instance, I used “vous” with a girl I was interested in for a long time though we met each other quite frequently, and were on friendly terms, because I liked it this way. I deliberatly switched to “tu” on our first actual date. At Truffaut time, not only it wouldn’t have been a quaint game, but I suspect that the “tu” would have had to wait until after the first kiss or the first night, and in 1920 it might not have been used even after the marriage.
Basically, do as you feel like. Many foreigners try to understand the “rules” but actually there aren’t clear rules, and even frenchmen like me sometimes are hesitant. It often comes naturally, but not always. Given the evolution over the years, I suspect “vous” might become rarer and rarer in the future, but I somehow regret it. Not fun.
On the internet, you might have noticed that “tu” is almost universally used in chatrooms, while “vous” tends to be used frequently on message boards, at least when adults are posting.
) lately, people who are comfortable with each other just switch to the informal form to appear more approachable and down-to-earth. When I was growing up about 2/3rds of my teachers addressed me using the familiar (although I had to address them using the formal back).