I have previously posted about being in a pretty unhappy 17 year maariage with a lazy guys who has been physically violent with son
( at times), complulsive gambler, lied, ran up $ 42, 000 debt and I am working extra to pay for it.

NOW... I finally have the confidence I need to take action. I have an appointment with a lawyer to look into a legal separation. I have not told him of my plans yet but suddenly he is over the top... nice, helpful, announced he has stopped drinking, even took the kids fishing 2 times! Sadly, I still want to separate... what is he thinking? He hasn't said a word to me about why the big change... Now I feel like I have no reason to push the issue of separation... is this a ploy on his part? How should I handle this? I asked him why the big change and he acted like he "always" acts like this. I don't trust him but now I feel like I can't continue my plan. My kids are so happy about their Dad's new behavior, I hate to break their hearts.

What do I do?

I just went back and read your post. Of course he is a nice guy now. He is also a user and an abuser. This nice behavior will not last. When you are looking at him with shining eyes of admiration for being the good daddy and he knows he is safe he will start up again.

Of course he will not say anyting to you about the change he will expect for you to go along with this just like you have with all of his cruel behaviors for the past 17 years. He's the one in charge here. Hell, look at all you have ALLOWed him to get away with INCLUDING hitting and choking your son. If you want to ride out this nice guy behavior so that you don't look like the bad one to the kids, go for it, what's a few more weeks compared to the last 17 years.

I agree with Bushg. Especially since he is in denial that he was not always the way he is now.
It shows that he is not being truthful with himself or you so you can not trust him. His new guy act will fade away just as quick as it happened so you need to keep your guard up and do not fall for it. Like Bushg said ride out the calm because if you try and divorce now you will be the bad guy, but first wrong move and you need to be prepared to make your stop at the lawyers right away.
My x could never keep up the act for more than 3 months at a time.

You've known him for 17 years; is he Dr. Jekyll or Mr. Hyde?