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Source: https://writers.stackexchange.com/a/48079
License name: CC BY-SA 4.0
License URL: https://creativecommons.org/licenses/by-sa/4.0/

As others have pointed out in comments, it's too much to cover in just 1 answer (I won't be shocked if this gets closed as too broad) but I will attempt to cover the basics. There are two things that are necessary.

1: consistency.

A character, unless if they are literally schizophrenic, should be consistent, even in change: that is, you should change while remaining consistent. If they suddenly switch from the Light Side to the Dark side it should be because of the values they've held the whole time. When Anakin turned evil it was because he didn't trust the Jedi and was arguably jealous of their power, which was shown throughout the whole film and even in the film before. Also to protect Padme which he clearly already cared about leading up to his change because he'd been trying to find out how to protect her the whole time. The urge to be more powerful and to protect Padme didn't come out of nowhere, it had been there the whole time, he just found a new way to fulfill it.

2: foreshadowing.

When Anakin turned evil it wasn't all at once, for example in Episode II when he killed the tuskin raiders (to be fair having seen the original trilogy we knew he'd turn evil anyways). When we learned Palpatine was evil it was no shock because it was already hinted at, for example, the fact that he knew an ancient Sith legend. Remember that some foreshadowing should seem insignificant at the time, but will make sense looking back after the fact.

Overall if you look at it, Anakin didn't change all _that_ much at the moment he agreed to become Palpatine's apprentice, he had already come to the edge of abandoning the Light Side, but it was here that he made the conscious decision to change. Kreia from Star Wars Kotor 2 sums it up very well

> It's a quiet thing, to fall, but far more devastating to admit it

Of course, this answer was specifically about turning from good to bad but any change of heart or allegiance would follow roughly the same roles.

Original score: 3