Some characters always have a plan, they’re always working out the angles, whether trying to work out what people are really thinking, watching the exits or figuring out the next con. This can all be included in an internal monologue, not necessarily all at once but we can build up the reader’s knowledge of how the character thinks so that even when a character doesn’t say they’re thinking it a reader will know.
To do this we need to establish that the character is a planner. We could do this all in one go, introducing the story with a scene where the character has to think their way out of a problem. This happens quite often in action-centric stories or films where we might begin with the character escaping or doing some form of mini-mission that introduces them. An example might be if a character is pinned down in a corner and is figuring out an escape route; another would be the character facing the villain at a poker table, a standard trope of Bond stories, and working out how the villain thinks using poker.
Instead of all at once we could introduce their mind set a bit at a time, we may even have them appear to be one way and turn out to be something else. A classic inversion might be the ‘innocent girl’ who goes into the alley and defeats the monster rather than being killed by the monster, think of Buffy the Vampire Slayer. There are two main ways of doing this; we may have an establishing scene, the woman walking into the alley and being followed, and the revelation, monster attacks her and she wins. Alternately we build it up slowly, which is where the internal monologue can really come into its own.
One of the ways to establish a differing mind set to the reader’s expectation is by dropping hints in the internal monologue. We may have a character walking along towards the alley, perhaps they hear a noise and instead of panicking they shrug it off and continue. Now this might not seem like using the internal monologue but in this case it would be the absence of something. A scared character might hear a noise and start trying to work out what it is, cycling through the possibilities in their internal monologue or trying to convince themselves it was nothing. However, this savvy monster slayer doesn’t think any of this, the reader might not think of it at the time but when it is revealed that this character walking into the alley is a monster slayer they realise it makes sense. The revelation fits because the character didn’t panic and the absence of panic acts to tell us something subliminally.
The assumption can be that when we’re writing a character who has a plan we have to tell the reader all about that plan, or we have to keep showing them figuring out the angles, which we can as long as we don’t overdo it and slow down the narrative or drift to far from the story. What can be forgotten in this is how much we can tell without telling, we don’t have to draw attention to the absence of a reaction for it to be noted, nor do we have to explain its absence. In the example of the monster slayer the answer is shown without being spoken because the character doesn’t panic. Why don’t they panic? They don’t panic because they’re a monster slayer used to meeting monsters in the dark.