Youtube actually provides a great example here; vloggers (those who produce video blogs) will disclose that they received products / services when reviewing them if they're trying to sell the product / service.
In other words, if they're actively pushing the watcher to do something (buy the product, use the service) knowing whether they're pushing because they genuinely like the thing, or because someone's paying them to push it is important. It can change our opinion of the product/service if we think the vlogger is mentioning it of their own free will, or because they have to in order to get paid.
Now to put it back in the perspective of your example; you're not trying to get your friend to buy stock in the company too, you're just having a casual conversation about something you enjoyed. No one's paying you to mention the movie and talk about how great it is. Therefore, you have no obligation to mention the stocks.
You only "need" (and even that's up to debate) to mention it when your investment in the company prevents you from giving an objective opinion / review of a product or service, or if the company is directly paying you specifically so that you mention them.