idiomatic way to describe an action, in which a main theme for a string of many similar subsequent actions was established

Question

What's the idiomatic way to describe an action, in which a main theme for a string of many similar subsequent actions was established?

On the very first lesson he told me that his hobby was inventing magic tricks, and that ____________________ many of our conversations with him in the following lessons of that year. We would talk about the magic tricks recently performed by some famed magicians on TV, about how they actually did those tricks, as well as about what kind of new tricks we ourselves would be able to come up with and impress others in our class.

1) laid down the topic for

2) gave rise to the topic of

3) predetermined the topic of

4) established the topic of

5) defined the topic of

6) ?

Answer 1

The phrase that first comes to mind for me here is set the stage.

He told me that his hobby was inventing magic tricks, and that set the stage for many of our (future) conversations.

The phrase comes from the idea of putting up the background scenery on the stage of a theatrical production to set the mood for the act to come. You can use it any time you want to talk about how some action created an opening that allowed future events to unfold. I think it works well in your example passage.

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Another option that comes to mind is open the door.

He told me that his hobby was inventing magic tricks, and that opened the door for many of our (future) conversations.

This one works similarly to "set the stage," but it tends to suggest that the thing that is going to happen as a result (whether that's the events that ultimately unfolded as a result or the new topic of discussion that came up or whatever) were somehow off limits or incapable of happening before the thing that opened the door happened.

For example, a conversation with someone with a lot of experience and connections in the industry you want to work in could open a door for you to a new career – the implication being that you couldn't realistically start that career on your own without that conversation having happened (maybe because you lacked experience or education in the field).

If you were talking about how one thing Person A said to Person B "opened the door" to a new topic of conversation, that would likely suggest that Person B couldn't or wouldn't bring up the subject themselves without Person A going there first, whether that be because it'd be impolite, or because the topic is very personal and the two aren't well acquainted, or whatever. So in the context of your example passage, "opened the door" might not be the best choice unless the magic trick hobby of whomever the narrator is referring to was some sort of secret, or the narrator happened to know that the person didn't like talking about it, or something else was preventing the narrator from broaching the subject up to that point.