Kamma is made in three ways: action, speech, and thoughts. All of these make Kamma. Actions, the action that you make also originated from a thought. The speech that you make also originated from a thought.
So, thought is part of your process of making Kamma no matter whether you make it in action, speech, or thought.
Now when you only have a thought and it stops, the Kamma is not so strong.
But if you have a thought and then you speak on the thought, the Kamma then becomes a stronger.
If you actually act on the thought, it gets even stronger.
A clear example is someone gets angry but they don't say anything, they don't do anything. Someone else gets angry and they yell. Someone else gets angry and they hit. You can easily see that the Kamma gets stronger as we apply our speech, and then stronger as we apply our actions. So, Kamma gets made on all three levels, but it is weakest if it only stays on the level of thought.
Now keep in mind Kamma is good and bad. With some mental formations, you can use them for goodness or you can use them not for goodness.
You sit down and you get angry, fine you make some negative Kamma.
You sit down and you reflect analytical thoughts about death, about Dukkha, whatever, you are going to make yourself wiser. That is going to be good Kamma. So, mental formations is not only a place of making Dukkha creations, it is also a place where we start developing our wisdom as well. So, it can make good Kamma.
You have heard the example I think Rosemary gives about a knife, using a knife for benefit or harm. We can use computers for benefit or harm, paper for benefit or harm. Yeah, you can start a nice fire with all this paper, couldn't you? So, like so many things, our mental formations as well, can be used for benefit or harm. It depends on how you use it as to what kind of Kamma you make.