One of the editorial comments (for which, read reasons for a rejection) which I often hear about is "There's too much introspection", or "The main character is too passive and doesn't do anything, just thinks." And although I see what they mean, I also see the writer's problem. I'd say that it's most common in character-driven novels, and perhaps it is a particular risk there, but the most recent version of this question was in fact from someone writing fantasy. The character had killed a person in self-defence, at the beginning of the novel. She must show remorse, but how to do that without make the character come across as a whingey drip?
Of course this is a version of a general problem, which is how to convey the full complexity and psychological depth of someone grappling with their situation, and then again when their situation changes (what do you mean, it doesn't change? You haven't got a story to write, if it doesn't) without running the story aground?
Where possible, Show/evoke how her emotional and psychological state in how she act and reacts in the world of the story (other characters, places, objects), rather than Telling/informing us what that state is.
Thinking is character-in-action too. Don't describe his thoughts, don't use his thinking to describe his situation for the reader, use it to evoke the way he grapples - or fails to grapple - with what he needs or wants. Concentrate on where you want the thinking to lead him, phrase by phrase, thought by thought - and write that linked chain that leads to somewhere. You need to work it out as clearly as you'd work out a paragraph evoking how he ambushes a thief and overpowers him.... or is overpowered. In other words, at the end of a paragraph of his thoughts about the impossibility of leaving his wife, he needs to have changed, by realising something he didn't realise before, or deciding to do something he wouldn't have done at the beginning.
Think character-in-(inter)action. It's more evocative to show how a table wobbles when she puts her plate down on it, and how the plate slides and only doesn't crash to the floor because it catches on the warped wood of the table top, than it is just to show the wonky table leg and crooked top, let alone to inform us that the table is badly made. Similarly, try to not to let the character-in-thinking-action lose touch with their physical setting. It's very rare just to sit and think. We mostly do it while carrying on with whatever needs doing, and even if we do make a cup of tea or scoop up the dog, and deliberately set to on some thinking... well, the tea gets spilt when you put it down clumsily, the dog gets impatient or distressed when you start to cry, or the doorbell rings just as you've finally laid out your dilemma in your mental lap but before you've decided what to do about it.
Think voice. We're talking about Psychic Distance here. The more strongly your character's voice comes out in their thinking, the more we'll have a sense of him or her as a real person, really doing something. Whether you're in first person or third, here is you chance to let the character's voice really flower, un-hampered by the dull business of everyday converse. And with our sense of this real, particular person doing something, will come a sense that another person might do something different; that this is subjective and particular. And that's when characters really come alive.
Think change in body as well as mind. Embody it in the character's physical state and behaviour, as well as evoking their thoughts.
Think bits and pieces. Of coures you're very clear (at least by the time you're revising) what the change is that this bit of thinking is leading to. But, having said that, don't forget that the change and character's realisation of the change may not come about in a single movement, as it were. What with dogs, and doorbells, and things we don't want to think, and things we genuinely don't realise ... that new understanding or new goal or desire, and the size of it, may only become apparent in bits and pieces and stages, not necessarily one huge Moment.
Think rear-view mirror. Remember how sometimes our awareness of how much we and our lives have changed only comes in retrospect. At the time we thought we were just dealing with today. The next day we look back on how we dealt with the drunk driver or the PTA meeting, and suddenly realise, "Golly, I'd never have reacted like that five years (or ten days) ago, before my father died/I got divorced/I ran that dog over." If you wrote the day-before PTA scene really vividly, then you won't need much sitting-at-the-tablet-with-coffee-staring-absently-out-of-the-window today to bring alive the whole new awareness.
Give the reader credit for intelligence, and also intuition. Just as you don't need to write every moment of the trip to his girlfriend's house with an engagement ring tucked in his pocket, you don't need to write every step of his thinking that led to him nipping down to H Samuel and take a risk on the size. Or put it this way: you may want or need to write them, so as to imagine them fully, but much of that will turn out to be the kind of scaffolding that you can take out in revision. I know I said that we need the chain of thinking, but some links are more equal than others, and those are what we need to feel as well as see in full psychological force.
And whatever you do, don't spill tea on the dog. Unless, of course, that makes you think that...