This can actually be a very complex question, and I'm not sure that all psychologists agree on the answer. The short version is that you use every function (introverted or extraverted) from time to time.
This is somewhat misleading though. When an INTP uses say Se, it won't be the same as when an ESP uses Se. So the next question is how they show up. Generally speaking, your primary function is the "hub" around which all other functions rotate. So consciously, your extraverted functions usually only show up when the primary function allows and for the purpose the primary function allows. I think that type development is less about "developing" functions as it is about getting the primary function to quit being so "monarchic" (as Jung might put it).
Think about it this way: let's say that your brain is a company with the primary function as the CEO. Of course, the CEO can't handle every situation by himself, so he hires employees (which are the other functions) to take care of things for him. In an undeveloped person, the CEO is a micromanager. It does everything it can to stop the other functions from doing anything it doesn't allow. So an INTP's extraverted functions will come out, but they'll be less alternate ways of looking at things and more the primary function's hired goons. Usually, your tertiary function is most willing to be the hired goon. It's the CEO's "yes man", and it oftentimes is tasked with making sure the secondary and inferior functions are following the CEO's orders. So the INTP might find themselves going back to old habits when they don't know what else to do because of tertiary Si.
At some point in time, the CEO gets stuck. It gets confronted with a situation it can't solve without trusting its employees. Thus, the CEO will eventually learn to let go a bit and trust its employees more and more.
Let's take the INTP as an example. When an ENP uses Ne, they do so to work with other people. INTPs are different. They'll typically use Ne as a way of protecting themselves from outside influence. If you watch House, consider the antics he goes through to get out of doing clinic duty: this is what Ne looks like when it's secondary to Ti. (And yes, I know that there's been plenty of debates about whether House is an ENTP, INTP, or INTJ. For the sake of argument, let's assume that House is an INTP. I don't think that necessarily detracts from my point.) At some point in time, the INTP will begin using Ne in other ways though. The INTP will start bouncing ideas off of other people instead of developing them internally.
To go back to the company analogy, some of the employees don't listen to the CEO. They have a nasty habit of doing their own thing when the CEO isn't paying attention. Your inferior function does this most often. The CEO knows this but can't fire them (remember, this is an analogy not real life :happy: ), so it simply convinces itself that they don't exist and becomes surprised every time they do something. This sounds completely illogical because it is. About the only thing that the CEO can really do is admit to itself that the employees exist, are problem employees, and that it will just have to deal with that.
So in the case of the INTP, you might notice Fe come out when you are frustrated or under stress. You might notice that you suddenly feel that nobody gives you the respect you deserve. At this point in time, the CEO thinks it's still in control but it isn't. Seeing through that illusion takes a great deal of effort.
So, like I said: you use every function from time to time and it's complicated.