From my personal experience, discussion with others and much reading, I find that the crucial turning point is when a Jew realizes that "Torah is actually and entirely true", i.e., it comes from God, has an unbroken chain of transmission to us through generations of interpreters, contains instructions/commands for all Jews, and needs to be taken seriously. Many serious non-observant Jews don't really see it this way, they see the Torah as a wise text that needs to be modernized or can be picked from.
Once someone shifts towards "this is all true, I need to observe all of it, and even if I'm not there today, I need to work to learn and observe more", then the person starts on a journey towards observance.
But these journeys are highly personal and often the product of long reflection & learning. They don't occur as a flash following one discussion. As such, you need to moderate your hopes to "switch on" people in one go. The most one can do is plant a few good questions, suggest further reading or discussions with experienced outreach professionals and be an attentive ear and supportive friend on the journey.
You also need to realize that people are frightened of change, wary of giving up things they like, worried of how they will look towards family and friends. These are very high obstacles to conquer.
I find that suggesting or offering books is an effective non-threatening starter on a journey of tshuva. Here are a few suggestions. These books will also provide you with great content to share